Prefix links with clang name for cpp support

When compiling cpp files, we need to call into clang++ instead of clang. This adds new symlinks that prefix 'clang-' and 'clang++-'. The python file then examines its path to determine whether it needs to invoke clang or clang++. It preserves backwards compatibility by making the clang- or clang++- prefix optional.

Tested by deleting clang-as-ios-dylib in xctool, symlinking my updates to the deleted path, and building + testing xctool.