Sema: prevent __declspec(naked) use on x64

MSDN (https://msdn.microsoft.com/en-us/library/h5w10wxs.aspx) indicates
that `__declspec(naked)` is only permitted on x86 and ARM targets.
Testing with cl does confirm this behaviour.  Provide a warning for use
of `__declspec(naked)` on x64.

llvm-svn: 299774