[Bug libstdc++/117221] Add smoketest for alternative compiler for libstdc++ headers (like CXX_UNDER_TEST)

[sjames at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117221

--- Comment #1 from Sam James <sjames at gcc dot gnu.org> ---
Tangentially: I don't know if we should consider running the whole testsuite
under Clang for this, if anybody's tried that before, or what.