Replace __cplusplus comparisons and dialect __has_feature checks with TEST_STD_VER.

This is a huge cleanup that helps make the libc++ test suite more portable.
Patch from STL@microsoft.com. Thanks STL!

llvm-svn: 272716
diff --git a/libcxx/test/std/strings/basic.string/string.require/contiguous.pass.cpp b/libcxx/test/std/strings/basic.string/string.require/contiguous.pass.cpp
index ca31ace..49773fe 100644
--- a/libcxx/test/std/strings/basic.string/string.require/contiguous.pass.cpp
+++ b/libcxx/test/std/strings/basic.string/string.require/contiguous.pass.cpp
@@ -41,7 +41,7 @@
     test_contiguous(S("1", A(5)));
     test_contiguous(S("1234567890123456789012345678901234567890123456789012345678901234567890", A(7)));
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     typedef min_allocator<char> A;
     typedef std::basic_string<char, std::char_traits<char>, A> S;