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

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1 @@
+constexpr float g8 = __builtin_nanf(u8"");
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AaronBallman wrote:

I think it's a bug that we don't reject this in C++ mode; `char8_t *` is a distinct type there which should not convert to `char *`. CC @cor3ntin @tahonermann for more opinions though.

I would prefer if we also diagnosed in C as well as C++. GCC accepts in C and that behavior is somewhat defensible given that `char8_t` is an alias to `unsigned char`, but I'd still like the diagnostic for the same reason Clang diagnoses this conversion in other cases: https://godbolt.org/z/z3d9ef7EE

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