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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Yup, that resolved the issue: https://lab.llvm.org/buildbot/#/builders/144/builds/2082 -- not certain why that builder has a different default value for `-std` though. Clang defaults to C17, but that builder seems to be defaulting to C99.

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