[clang][Interp] Fix CFStringMakeConstantString etc. evaluation

We're ultimately expected to return an APValue simply pointing to
the CallExpr, not any useful value. Do that by creating a global
variable for the call.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -3562,6 +3562,17 @@ bool ByteCodeExprGen<Emitter>::VisitBuiltinCallExpr(const CallExpr *E) {
   if (!Func)
     return false;
 
+  // For these, we're expected to ultimately return an APValue pointing
+  // to the CallExpr. This is needed to get the correct codegen.
+  unsigned Builtin = E->getBuiltinCallee();
+  if (Builtin == Builtin::BI__builtin___CFStringMakeConstantString ||
+      Builtin == Builtin::BI__builtin___NSStringMakeConstantString ||
+      Builtin == Builtin::BI__builtin_function_start) {
+    if (std::optional<unsigned> GlobalOffset = P.createGlobal(E))
+      return this->emitGetPtrGlobal(*GlobalOffset, E);
+    return false;
+  }
+
   QualType ReturnType = E->getType();
   std::optional<PrimType> ReturnT = classify(E);
 

clang/lib/AST/Interp/InterpBuiltin.cpp

@@ -1329,8 +1329,6 @@ bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const Function *F,
     break;
 
   case Builtin::BI__builtin_launder:
-  case Builtin::BI__builtin___CFStringMakeConstantString:
-  case Builtin::BI__builtin___NSStringMakeConstantString:
     if (!noopPointer(S, OpPC, Frame, F, Call))
       return false;
     break;

clang/test/CodeGen/cfstring.c

@@ -1,6 +1,7 @@
 // REQUIRES: x86-registered-target
 
 // RUN: %clang_cc1 -triple x86_64-macho -emit-llvm %s -o %t
+// RUN: %clang_cc1 -triple x86_64-macho -emit-llvm %s -o %t -fexperimental-new-constant-interpreter
 
 // Check that the backing store of CFStrings are constant with the
 // -fwritable-strings flag.