[llvm] [compiler-rt] [test][msan] Precommit tests for vararg improvements (PR #72612)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

> Why do we need separate ll files for ARM and X86? Is there a way to combine those?

VarArg is ABI specific
IR is different even before sanitizer instrumentation.

https://github.com/llvm/llvm-project/pull/72612