Re-land "[MS] Overhaul how clang passes overaligned args on x86_32"

This brings back 2af74e27 and reverts
eaabaf7e.

The changes were correct, the code that was broken contained an ODR
violation that assumed that these types are passed equivalently:
  struct alignas(uint64_t) Wrapper { uint64_t P };
  void f(uint64_t p);
  void f(Wrapper p);

MSVC does not pass them the same way, and so clang-cl should not pass
them the same way either.