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
Xiangling Liao