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!


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@272716 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.ops/string.accessors/c_str.pass.cpp

@@ -40,7 +40,7 @@ int main()
     test(S("abcdefghij"));
     test(S("abcdefghijklmnopqrst"));
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
     test(S(""));

test/std/strings/basic.string/string.ops/string.accessors/get_allocator.pass.cpp

@@ -34,7 +34,7 @@ int main()
     test(S("abcdefghij", A(2)), A(2));
     test(S("abcdefghijklmnopqrst", A(3)), A(3));
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     typedef min_allocator<char> A;
     typedef std::basic_string<char, std::char_traits<char>, A> S;