[clang] fix bug that undefined internal is a warning only for -pedantic-errors (PR #98016)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I think adding `-std=c11` to the `RUN` line should help; it at least removes the "is a C11 extension" diagnostics. (I guess this builder isn't defaulting to C17 for some reason?)

https://github.com/llvm/llvm-project/pull/98016