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.

[unittest][codegen] Avoid memory leak in recently added FindDebugLocTest unittest

Need to finalize the DIBuilder to avoid leak sanitizer errors
like this:

Direct leak of 48 byte(s) in 1 object(s) a

[unittest][codegen] Avoid memory leak in recently added FindDebugLocTest unittest

Need to finalize the DIBuilder to avoid leak sanitizer errors
like this:

Direct leak of 48 byte(s) in 1 object(s) allocated from:
#0 0x55c99ea1761d in operator new(unsigned long)
#1 0x55c9a518ae49 in operator new
#2 0x55c9a518ae49 in llvm::MDTuple::getImpl(...)
#3 0x55c9a4f1b1ec in getTemporary
#4 0x55c9a4f1b1ec in llvm::DIBuilder::createFunction(...)

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[CodeGen] Fix for MachineBasicBlock::rfindDebugLoc(instr_rend())

Make sure we do not crash in rfindDebugLoc when starting at
instr_rend(). Solution is to see it as we start one MI before the
first M

[CodeGen] Fix for MachineBasicBlock::rfindDebugLoc(instr_rend())

Make sure we do not crash in rfindDebugLoc when starting at
instr_rend(). Solution is to see it as we start one MI before the
first MI, so we can start searching forward at instr_begin()
instead.

This behavior is similar to how findPrevDebugLoc(instr_end()) works.

Differential Revision: https://reviews.llvm.org/D150577

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[CodeGen] Add unittest for findDebugLoc, rfindDebugLoc, findPrevDebugLoc and rfindPrevDebugLoc. NFC

- Add some unittests for the findDebugLoc, rfindDebugLoc,
findPrevDebugLoc and rfindPrevDebugLoc

[CodeGen] Add unittest for findDebugLoc, rfindDebugLoc, findPrevDebugLoc and rfindPrevDebugLoc. NFC

- Add some unittests for the findDebugLoc, rfindDebugLoc,
findPrevDebugLoc and rfindPrevDebugLoc helpers in MachineBasicBlock.

- Clean up code comments and code formatting related to the functions
mentioned above.

This was extracted as a pre-commit to D150577, adn some of the tests
are commented out since they would crash/assert in a rather
uncontrolled way.

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