[NFC] Renaming PackStack to AlignPackStack

This patch renames PackStack and related variable names to also contain align across Clang.
As it is right now, Clang already uses one stack to record the information from both #pragma
align and #pragma pack. Leaving it as PackStack is confusing, and could cause people to
ignore #pragma align when developing code that interacts with PackStack.

Differential Revision: https://reviews.llvm.org/D93901