[NFC] Fix incorrect (but unreachable) LIT error message

It is unreachable because we test that the cxx_stdlib_under_test is
in the supported set of libraries elsewhere. Furthermore, this code
relied on the `use_stdlib_type`, which is never defined.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348867 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed