Make sema and codegen allow __builtin___CFStringMakeConstantString as a valid
constant lvalue.  Implement this in codegen by moving the code out of CGBuiltin
into EmitConstantExpr.

llvm-svn: 57163
diff --git a/clang/test/CodeGen/cfstring.c b/clang/test/CodeGen/cfstring.c
index ef42d06..7d7edec 100644
--- a/clang/test/CodeGen/cfstring.c
+++ b/clang/test/CodeGen/cfstring.c
@@ -4,3 +4,8 @@
 void f() {
   CFSTR("Hello, World!");
 }
+
+// rdar://6248329
+void *G = CFSTR("yo joe");
+
+