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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I think the test cases should all be combined into a single file -- we don't stop issuing diagnostics after the first one, so it should be fine to put them all together into one file.

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