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 C64


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201453 91177308-0d34-0410-b5e6-96231b3b80d8