Fix __alignof__ of global variables on SystemZ

SystemZ prefers to align all global variables to two bytes, which is
implemented by setting the TargetInfo member MinGlobalAlign.

However, for compatibility with existing compilers this should *not*
change the ABI alignment value as retrieved via __alignof__, which
it currently does.

This patch fixes the issue by having ASTContext::getDeclAlign ignore
the MinGlobalAlign setting in the ForAlignof case.

Since SystemZ is the only platform setting MinGlobalAlign, this should
cause no change for any other target.

llvm-svn: 235395