test: Decorators for pr17910

A number of tests fail to build on FreeBSD because the test build script
defaults to libstdc++ for clang.  On FreeBSD the libstdc++ is rather old
and libc++ should be used instead.

(These tests previously had an @expectedFailureFreeBSD decorator for
pr16696, the umbrella PR for the lack of threaded inferior support on
FreeBSD.  The work to add that support will be committed soon.)

llvm-svn: 196101