[libc++] NFC: Clean up a lot of old Lit features

The libc++ test suite has a lot of old Lit features used to XFAIL tests
and mark them as UNSUPPORTED. Many of them are to workaround problems on
old compilers or old platforms. As time goes by, it is good to go and
clean those up to simplify the configuration of the test suite, and also
to reflect the testing reality. It's not useful to have markup that gives
the impression that e.g. clang-3.3 is supported, when we don't really
test on it anymore (and hence several new tests probably don't have the
necessary markup on them).

libcxx/test/std/strings/basic.string/string.modifiers/string_assign/T_size_size.pass.cpp

@@ -11,11 +11,6 @@
 // template <class T>
 //    basic_string& assign(const T& t, size_type pos, size_type n=npos); // C++17
 
-// 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 <cassert>

libcxx/test/std/strings/basic.string/string.modifiers/string_assign/string_size_size.pass.cpp

@@ -12,11 +12,6 @@
 //   assign(const basic_string<charT,traits>& str, size_type pos, size_type n=npos);
 // the =npos was added for C++14
 
-// 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 <cassert>