[clang] [Clang] Fix assertion failure when passing wide string literal to __builtin_nanf (PR #205452)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,3 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
----------------
AaronBallman wrote:

Given there are type differences between C and C++ for string literals, it probably makes sense to add a `RUN` line with `-x c++` so we test C++ as well.

https://github.com/llvm/llvm-project/pull/205452