AArch64: support funclets in fastcall and swift_call

Functions annotated with `__fastcall` or `__attribute__((__fastcall__))`
or `__attribute__((__swiftcall__))` may contain SEH handlers even on
Win64.  This matches the behaviour of cl which allows for
`__try`/`__except` inside a `__fastcall` function.  This was detected
while trying to self-host clang on Windows ARM64.

llvm-svn: 348337