[ABI][NFC] Fix the confusion of ByVal and ByRef argument names

The second argument of getNaturalAlignIndirect() was `bool ByRef`, but
the implementation was just delegating to getIndirect() with `ByRef`
passed unchanged to `bool ByVal` parameter of getIndirect().

Fix a couple of /*ByRef=*/ comments as well.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D85113