Overhaul the TargetMachine and LLVMTargetMachine Classes (#111234)

Following discussions in #110443, and the following earlier discussions
in https://lists.llvm.org/pipermail/llvm-dev/2017-October/

Overhaul the TargetMachine and LLVMTargetMachine Classes (#111234)

Following discussions in #110443, and the following earlier discussions
in https://lists.llvm.org/pipermail/llvm-dev/2017-October/117907.html,
https://reviews.llvm.org/D38482, https://reviews.llvm.org/D38489, this
PR attempts to overhaul the `TargetMachine` and `LLVMTargetMachine`
interface classes. More specifically:
1. Makes `TargetMachine` the only class implemented under
`TargetMachine.h` in the `Target` library.
2. `TargetMachine` contains target-specific interface functions that
relate to IR/CodeGen/MC constructs, whereas before (at least on paper)
it was supposed to have only IR/MC constructs. Any Target that doesn't
want to use the independent code generator simply does not implement
them, and returns either `false` or `nullptr`.
3. Renames `LLVMTargetMachine` to `CodeGenCommonTMImpl`. This renaming
aims to make the purpose of `LLVMTargetMachine` clearer. Its interface
was moved under the CodeGen library, to further emphasis its usage in
Targets that use CodeGen directly.
4. Makes `TargetMachine` the only interface used across LLVM and its
projects. With these changes, `CodeGenCommonTMImpl` is simply a set of
shared function implementations of `TargetMachine`, and CodeGen users
don't need to static cast to `LLVMTargetMachine` every time they need a
CodeGen-specific feature of the `TargetMachine`.
5. More importantly, does not change any requirements regarding library
linking.

cc @arsenm @aeubanks

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[unittests] Use {} instead of std::nullopt to initialize empty ArrayRef (#109388)

Follow up to #109133.

[IRBuilder] Don't include Module.h (NFC) (#97159)

This used to be necessary to fetch the DataLayout, but isn't anymore.

[MC] Do not copy MCInstrDescs. NFC.

Avoid copying MCInstrDesc instances because a future patch will change
them to find their implicit operands and operand info array based on
their own "this" point

[MC] Do not copy MCInstrDescs. NFC.

Avoid copying MCInstrDesc instances because a future patch will change
them to find their implicit operands and operand info array based on
their own "this" pointer, so it will only work for MCInstrDescs in the
TargetInsts table, not for a copy of an MCInstrDesc at a different
address.

Differential Revision: https://reviews.llvm.org/D142214

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[llvm/unittests] Use std::nullopt instead of None (NFC)

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated. The intent is to reduce
the am

[llvm/unittests] Use std::nullopt instead of None (NFC)

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated. The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

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[Codegen][tablgen][NFC] Allow meta instruction to be target dependent

An instruction is a meta-instruction if it doesn't produce any output
in the form of executable instructions. So in the concept,

[Codegen][tablgen][NFC] Allow meta instruction to be target dependent

An instruction is a meta-instruction if it doesn't produce any output
in the form of executable instructions. So in the concept, a
meta-instruction does not have to be target independent.

Before this patch, `isMetaInstruction` is implemented by checking the
opcode of the instruction, add we have no way to add target dependent
opcode to the list, which does not make sense.

After this patch, a bit `isMeta` is added for class `Instruction` in
tablegen, which is used to indicate whether it's a meta instruction.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D121600

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Move TargetRegistry.(h|cpp) from Support to MC

This moves the registry higher in the LLVM library dependency stack.
Every client of the target registry needs to link against MC anyway to
actually us

Move TargetRegistry.(h|cpp) from Support to MC

This moves the registry higher in the LLVM library dependency stack.
Every client of the target registry needs to link against MC anyway to
actually use the target, so we might as well move this out of Support.

This allows us to ensure that Support doesn't have includes from MC/*.

Differential Revision: https://reviews.llvm.org/D111454

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[LexicalScopesTest] Add missing IRBuilder.h include (NFC)

This currently depends on a transitive include via TargetLowering.h.

[docs][unittest][Go][StackProtector] Migrate deprecated DebugInfo::get to DILocation::get

[DebugInfo] Re-implement LexicalScopes dominance method, add unit tests

Way back in D24994, the combination of LexicalScopes::dominates and
LiveDebugValues was identified as having worst-case quadra

[DebugInfo] Re-implement LexicalScopes dominance method, add unit tests

Way back in D24994, the combination of LexicalScopes::dominates and
LiveDebugValues was identified as having worst-case quadratic complexity,
but it wasn't triggered by any code path at the time. I've since run into a
scenario where this occurs, in a very large basic block where large numbers
of inlined DBG_VALUEs are present.

The quadratic-ness comes from LiveDebugValues::join calling "dominates" on
every variable location, and LexicalScopes::dominates potentially touching
every instruction in a block to test for the presence of a scope. We have,
however, already computed the presence of scopes in blocks, in the
"InstrRanges" of each scope. This patch switches the dominates method to
examine whether a block is present in a scope's InsnRanges, avoiding
walking through the whole block.

At the same time, fix getMachineBasicBlocks to account for the fact that
InsnRanges can cover multiple blocks, and add some unit tests, as Lexical
Scopes didn't have any.

Differential revision: https://reviews.llvm.org/D73725

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