[OpenCL] Handle TypeExtensions in OpenCLBuiltinFileEmitter

Until now, any types that had TypeExtensions attached to them were not
guarded with those extensions.  Extend the OpenCLBuiltinFileEmitter
such that all required extensions are emitted for the types of a
builtin function.

The `clang-tblgen -gen-clang-opencl-builtin-tests` emitter will now
produce e.g.:

  #if defined(cl_khr_fp16) && defined(cl_khr_fp64)
  half8 test11802_convert_half8_rtp(double8 arg1) {
    return convert_half8_rtp(arg1);
  }
  #endif // TypeExtension

Differential Revision: https://reviews.llvm.org/D120262