Pass cxxCompiler extraCLIFlags to swiftc once supported

[rauhul]

Description

SwiftPM currently passes cCompiler extraCLIFlags to swiftc when building a swift target by prefixing the arguments with -Xcc. This is useful as it ensures the clang-importer and clang itself to build C targets with the same set of flags.

A similar need exists for the clang-importer and clang++. They should use the same set of flags. This however is not yet possible as swiftc does have a -Xcxx flag which could prefix the cxxCompiler extraCLIFlags. The -Xcc prefix also cannot be used as c++ flags cannot all safely be passed to clang (in C mode).

Once swiftc supports -Xcxx spm should adopt it and pass cxxCompiler extraCLIFlags to swiftc.