[clang] Add test for C++ DR2390

DR2390 clarifies that the argument to __has_cpp_attribute() needs to be
macro-expanded. Clang already supports this and tests it explicitly in
clang/test/Preprocessor/has_attribute.cpp.

Copy the test over to clang/test/CXX/drs/ so the make_cxx_drs script
picks it up.

Differential Revision: https://reviews.llvm.org/D119230