[clang] [CIR] Add support for function linkage and visibility (PR #145600)

[Erich Keane via cfe-commits]

================
@@ -406,14 +406,34 @@ void CIRGenModule::emitGlobalFunctionDefinition(clang::GlobalDecl gd,
                                /*DontDefer=*/true, ForDefinition);
   }
 
+  // Already emitted.
+  if (!funcOp.isDeclaration())
----------------
erichkeane wrote:

So this was a little jarring to me... any chance we could add a `isDefinition` to `cir::FuncOp` (or `isDefined`?) and use that instead?  Took me 2-3 reads to figure out What is going on here :)

https://github.com/llvm/llvm-project/pull/145600