[libcxx] Reintroduce UNSUPPORTED annotation for strstreambuf overflow test

This is a revert of r347421, except I'm using the with_system_cxx_lib
lit feature instead of availability to mark the test as unsupported
(because the problem is a bug in the dylib itself). In r347421, I said
I wasn't able to reproduce the issue and that's why I was removing it:
this was because I ran lit slightly wrong. The problem mentioned really
exists.

llvm-svn: 347475

libcxx/test/std/depr/depr.str.strstreams/depr.strstreambuf/depr.strstreambuf.members/overflow.pass.cpp

@@ -13,6 +13,10 @@
 
 // int overflow(int c);
 
+// There was an overflow in the dylib on older macOS versions
+// UNSUPPORTED: with_system_cxx_lib=macosx10.8
+// UNSUPPORTED: with_system_cxx_lib=macosx10.7
+
 #include <iostream>
 #include <string>
 #include <strstream>