[clang][ARM] Add name-mangling test for direct __fp16 arguments.

`clang/test/CodeGenCXX/fp16-mangle.cpp` tests pointers to __fp16, but
if you give the `-fallow-half-arguments-and-returns` option, then
clang can also leave an __fp16 unmodified as a function argument or
return type. This regression test checks the name-mangling of that.

Reviewed By: miyuki

Differential Revision: https://reviews.llvm.org/D85010