[Win64] Only treat some functions as having the Win64 convention

All the usual X86 target-specific conventions are collapsed to the
normal Win64 convention, but the custom conventions like GHC and webkit
should not be.

Previously we would assume that the caller allocated 32 bytes of shadow
space for us, which is not how webkit_jscc or other custom conventions
are supposed to work.

Based on a patch by peavo@outlook.com.

Fixes PR24051.

llvm-svn: 241725