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

[Bruno Cardoso Lopes 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.
----------------
bcardosolopes wrote:

What we get in this case is:
```
module ...  {
  cir.global  external dsolocal @y = #cir.int<22> : !s32i {alignment = 4 : i64} loc(#loc3)
} loc(#loc)
#loc = loc("a.cpp":0:0)
#loc1 = loc("a.cpp":3:1)
#loc2 = loc("a.cpp":3:9)
#loc3 = loc(fused[#loc1, #loc2])
```

> which location does MLIR expect to work with?

Whatever we tell it should hold. We usually take the presumed (expanded) loc (from `getSourceRange` or begin/end):

```
mlir::Location CIRGenFunction::getLoc(SourceLocation SLoc) {
  // Some AST nodes might contain invalid source locations (e.g.
  // CXXDefaultArgExpr), workaround that to still get something out.
  if (SLoc.isValid()) {
    const SourceManager &SM = getContext().getSourceManager();
    PresumedLoc PLoc = SM.getPresumedLoc(SLoc);
    StringRef Filename = PLoc.getFilename();
    return mlir::FileLineColLoc::get(builder.getStringAttr(Filename),
                                     PLoc.getLine(), PLoc.getColumn());
  } else {
    // Do our best...
    assert(currSrcLoc && "expected to inherit some source location");
    return *currSrcLoc;
  }
}
```
My take is that this is good enough for CIRGen purposes and can/should probably be improved when we start working on emitting debug information (where we can go for non-presumed locs if it makes sense), before that it feels designing prematurely to me.

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