[clang] [FreeBSD] Support -stdlib=libstdc++ (PR #126302)

[John Baldwin via cfe-commits]

bsdjhb wrote:

> It's not missing as in like someone forgot. It is intentional. Same goes for FreeBSD and OpenBSD's Drivers. The base OS uses libc++ and anything else is not supported.

You could use it with a libstdc++ from ports.  GCC supports both variants for the same reason.  FreeBSD's default GCC ports include libstdc++ which is used for 3rd party packages built with GCC instead of clang.  I also suggested this approach to Alex for fixing the test failure.  I'd really rather the option "work".  I depend on the option working in the reverse when using GCC to build FreeBSD's base system (that is, -stdlib=c++).  Someone using -stdlib=stdc++ should get what they have explicitly asked for.

https://github.com/llvm/llvm-project/pull/126302