[clang] [Modules] Fix using `va_list` with modules and a precompiled header. (PR #100837)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

Looks like need something like `// REQUIRES: x86-registered-target` in the test. But still need to check the correct approach in other tests.

https://github.com/llvm/llvm-project/pull/100837