[clang][libcxx] renames `__remove_reference`

libc++ prior to LLVM 15 has a bug in it due to it excluding
`remove_reference_t` when `__remove_reference` is available as a
compiler built-in. This went unnoticed until D116203 because it wasn't
available in any compiler.

To work around this, we're renaming `__remove_reference` to
`__remove_reference_t`.

TEST=Tested locally, tested using emscripten