Fix warnings building swift/ABI with MSVC

[hughbe]

• Your bog-standard unhandled control path warnings. Can't use llvm_unreachable as this is used in the stdlib where we can't link LLVM

[compnerd]

I think that covering the switch with the one missing value with an llvm_unreachable is nicer.

[hughbe]

@compnerd we can't use llvm_unreachable - this is part of the stdlib and I get linker errors for unresolved external llvn_unreachable_internal when compiling the stdlib if we us unreachable.

[hughbe]

Will update to use swift_unreachable when #6025 is merged

[compnerd]

@hughbe #6205 is merged

[hughbe]

Thanks @compnerd. I've updated this :)

[compnerd]

@swift-ci please test and merge

[hughbe]

@compnerd tests passed. can you merge manually, thanks

[compnerd]

@swift-ci please smoke test OSX platform

[compnerd]

Just being a bit more paranoid :)

[compnerd]

@swift-ci please smoke test OS X platform

[hughbe]

@jrose-apple can we merge this in, then I'll rename swift_unreachable to swift_runtime_unreachable. Thanks!