Tooling: silence -Wreturn-type warning
Tooling: silence -Wreturn-type warning

The switch should be covered.  However, older versions of gcc and clang as well
as MSVC do not handle the covered switch and emit a warning.  Silence the
warning by adding an unreachable.  NFC.

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang