[CIR][CodeGen] Store the old CIRGenFunction when popping to a new one

We diverge from CodeGen here by delaying the function emission that
happens for a global variable. However, due to situations where a global
can be emitted while building out a function the old CGF might not be
invalid. So we need to store it here just in case.

Reviewers: bcardosolopes, smeenai

Reviewed By: smeenai

Pull Request: #1023

Author: lanza

clang/lib/CIR/CodeGen/CIRGenCXX.cpp

@@ -18,6 +18,7 @@
 
 #include "clang/AST/GlobalDecl.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/SaveAndRestore.h"
 #include <cassert>
 
 using namespace clang;
@@ -337,7 +338,7 @@ void CIRGenModule::buildCXXGlobalVarDeclInit(const VarDecl *varDecl,
 
   assert(varDecl && " Expected a global declaration!");
   CIRGenFunction cgf{*this, builder, true};
-  CurCGF = &cgf;
+  llvm::SaveAndRestore<CIRGenFunction*> savedCGF(CurCGF, &cgf);
   CurCGF->CurFn = addr;
 
   CIRGenFunction::SourceLocRAIIObject fnLoc{cgf,
@@ -421,6 +422,4 @@ void CIRGenModule::buildCXXGlobalVarDeclInit(const VarDecl *varDecl,
         builder.create<mlir::cir::YieldOp>(addr->getLoc());
     }
   }
-
-  CurCGF = nullptr;
 }