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

[Aaron Ballman via cfe-commits]

================
@@ -52,10 +62,33 @@ class CIRGenModule : public CIRGenTypeCache {
   /// A "module" matches a c/cpp source file: containing a list of functions.
   mlir::ModuleOp theModule;
 
+  clang::DiagnosticsEngine &diags;
+
   const clang::TargetInfo ⌖
 
 public:
+  mlir::ModuleOp getModule() const { return theModule; }
+
+  /// Helpers to convert Clang's SourceLocation to an MLIR Location.
----------------
AaronBallman wrote:

I don't know the details about CIR, so it's hard for me to say for certain. Maybe an example will help tease this out? Given:
```
#define ten 10
#define foo(x) (x + ten)
int y = foo(12);
```
you get source locations like: https://godbolt.org/z/oG7fq9b1K Notice how you get `Loc=<<source>:3:9 <Spelling=<source>:2:16>>` or similar where the source location of the final expanded token and the spelling location of where the pre-preprocessed token lives are different -- which location does MLIR expect to work with? It could work with the spelling location (where tokens physically reside in the source file), or it could work with the expansion locations (where the tokens logically reside in the source file after preprocessing), or it could be ambivalent about which form it uses and will happily work with whatever.

https://github.com/llvm/llvm-project/pull/113483