[clang][CodeGen] Use memory type representation in `va_arg`

Some types (e.g. `_Bool`) have different scalar and memory representations. CodeGen for `va_arg` didn't take this into account, leading to an assertion failures with different types.

This patch makes sure we use memory representation for `va_arg`.

Reviewed By: ahatanak

Differential Revision: https://reviews.llvm.org/D118904