Clean up string's deduction guides tests. Mark old versions of clang as...

Clean up string's deduction guides tests. Mark old versions of clang as unsupported, b/c they don't have deduction guides, even in C++17 mode

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324619 91177308-0d34-0410-b5e6-96231b3b80d8