[clang] Add `__has_extension ()` for C++11 features

Add `__has_extension (cxx_defaulted_functions)` and
`__has_extension (cxx_default_function_template_args)` since they are
accepted in C++98 mode as extensions.

Fixes https://github.com/llvm/llvm-project/issues/61758

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D154334