Build swift-frontend as the primary Swift binary, and have
swift/swiftc/etc. symlink over to it. This is a step toward allowing
swift-driver to replace the swift and swiftc binaries.
Clang 8 or 9 seems to have changed from EXE to exe. Allow both
capitalizations (which is not important in Windows) as a workaround.
Maybe in the future we can remove the uppercase option.
