build: limit clang flags to C++ (NFC)
build: limit clang flags to C++ (NFC)

This adjusts the application of the flags to just the C++ parts of the code,
enabling moving to CMake 3.15.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild