[libc++] libc++ doesn't compile with gcc 15

[killcerr]

libc++ doesn't compile with GCC 15 because it comes with new GCC __decay built-in which is incompatible with libc++.
similar issue
the bug file

[frobtech]

There are numerous other new built-ins that have the same issues. __decay is just the most widely-used of them.
#137704 lists all of them.