clang/test/CXX/drs/dr5xx.cpp: Fix up assumption of thiscall. It affects not x64 but x86.

Note, i686-cygwin doesn't use thiscall.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217234 91177308-0d34-0410-b5e6-96231b3b80d8