[libc++] Mark several tests as XFAIL on macosx10.7

Those tests fail when linking against a new dylib but running against
macosx10.7. I believe this is caused by a duplicate definition of the
RTTI for exception classes in libc++.dylib and libc++abi.dylib, but
this matter still needs some investigation.

This issue was not caught previously because all the tests always linked
against the same dylib used for running (because LIT made it impossible
to do otherwise before r349171).

rdar://problem/46809586

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@354940 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.cons/substr.pass.cpp

@@ -16,6 +16,11 @@
 //              size_type pos,
 //              const Allocator& a = Allocator());
 
+// When back-deploying to macosx10.7, the RTTI for exception classes
+// incorrectly provided by libc++.dylib is mixed with the one in
+// libc++abi.dylib and exceptions are not caught properly.
+// XFAIL: with_system_cxx_lib=macosx10.7
+
 #include <string>
 #include <stdexcept>
 #include <algorithm>