[CIR] Fix warning in CIRGenAction.cpp (#118389)

Fix a compiler warning in `CIRGenConsumer::HandleTranslationUnit` in
`clang/lib/CIR/FrontendAction/CIRGenAction.cpp`. The warning was about a
`defau

[CIR] Fix warning in CIRGenAction.cpp (#118389)

Fix a compiler warning in `CIRGenConsumer::HandleTranslationUnit` in
`clang/lib/CIR/FrontendAction/CIRGenAction.cpp`. The warning was about a
`default:` section in a switch statement that already covered all the
values of an enum. Delete the `default:` section.

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[CIR] Call code gen; create empty cir.func op (#113483)

Finish hooking up ClangIR code gen into the Clang control flow,
initializing enough that basic code gen is possible.

Add an almost empty `

[CIR] Call code gen; create empty cir.func op (#113483)

Finish hooking up ClangIR code gen into the Clang control flow,
initializing enough that basic code gen is possible.

Add an almost empty `cir.func` op to the ClangIR dialect. Currently the
only property of the function is its name. Add the code necessary to
code gen a cir.func op.

Create essentially empty files
clang/lib/CIR/Dialect/IR/{CIRAttrs.cpp,CIRTypes.cpp}. These will be
filled in later as attributes and types are defined in the ClangIR
dialect.

(Part of upstreaming the ClangIR incubator project into LLVM.)

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[CIR] Build out AST consumer patterns to reach the entry point into CIRGen

Build out the necessary infrastructure for the main entry point into
ClangIR generation -- CIRGenModule. A set of boilerpla

[CIR] Build out AST consumer patterns to reach the entry point into CIRGen

Build out the necessary infrastructure for the main entry point into
ClangIR generation -- CIRGenModule. A set of boilerplate classes exist
to facilitate this -- CIRGenerator, CIRGenAction, EmitCIRAction and
CIRGenConsumer. These all mirror the corresponding types from LLVM
generation by Clang's CodeGen.

The main entry point to CIR generation is
`CIRGenModule::buildTopLevelDecl`. It is currently just an empty
function. We've added a test to ensure that the pipeline reaches this
point and doesn't fail, but does nothing else. This will be removed in
one of the subsequent patches that'll add basic `cir.func` emission.

This patch also re-adds `-emit-cir` to the driver. lib/Driver/Driver.cpp
requires that a driver flag exists to facilirate the selection of the
right actions for the driver to create. Without a driver flag you get
the standard behaviors of `-S`, `-c`, etc. If we want to emit CIR IR
and, eventually, bytecode we'll need a driver flag to force this. This
is why `-emit-llvm` is a driver flag. Notably, `-emit-llvm-bc` as a cc1
flag doesn't ever do the right thing. Without a driver flag it is
incorrectly ignored and an executable is emitted. With `-S` a file named
`something.s` is emitted which actually contains bitcode.

Reviewers: AaronBallman, MaskRay, bcardosolopes

Reviewed By: bcardosolopes, AaronBallman

Pull Request: https://github.com/llvm/llvm-project/pull/91007

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