Rollup merge of #155622 - folkertdev:va-arg-llvm-fixes, r=tgross35

c-variadic: `va_arg` fixes

tracking issue: rust-lang/rust#44930

extracts some generic LLVM `va_arg` fixes from rust-lang/rust#152576 and rust-lang/rust#155429.

r? tgross35

Author: JonathanBrouwer