[compiler-rt] [rtsan] Fix va_args handling in open functions (PR #108291)

[Chris Apple via llvm-commits]

cjappl wrote:

Hmm, as a counter-argument to this PR, it seems like tsan does the same thing that we do now, always unpacking the arg:

https://github.com/llvm/llvm-project/blob/c2b93e0671d8cfd6b1a24c6e1d7be290125b8974/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp#L1664-L1675


https://github.com/llvm/llvm-project/pull/108291