MS ABI: Fix some layout tests

Some lines intended to be used for testing x86_64 ABI compatibility were
not firing because lines were annotated with the wrong FileCheck prefix:
X64 vs x64

N.B. Changes beyond just changing x64 to X64 were made, presumably
because other parts of the layout engine have changed.  I've verified
the changes to make sure that MSVC creates a compatible layout.

llvm-svn: 200670