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.)

Followup commit:

Improve comments, variable names, and formatting.  Some other code
cleanup.

Fix bugs in the creation of the `cir.func` op: add it to the module, and
correctly store its name.

Update the hello.c test to check that a `cir.func` op is generated.

Followup change responding to code review comments.  There are no
behavior changes, just small cleanup.

More followup responding to review comments.  Make
`CIRGenerator::getModule()` a `const` member function. Call
`loadDialect` instead of `getOrLoadDialect` to load MLIR dialects.  Fix
a typo and remove unnecessary curly braces.

Improve the comment about converting `SourceLocation` to
`mlir::Location`, being explicit that the presumed location is
converted.

Part of llvm#113483 [CIR] Call code gen; create empty cir.func op