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/support/test_macros.h

@@ -35,6 +35,7 @@
 #endif
 
 /* Make a nice name for the standard version */
+#ifndef TEST_STD_VER
 #if  __cplusplus <= 199711L
 # define TEST_STD_VER 3
 #elif __cplusplus <= 201103L
@@ -44,6 +45,7 @@
 #else
 # define TEST_STD_VER 99    // greater than current standard
 #endif
+#endif
 
 /* Features that were introduced in C++14 */
 #if TEST_STD_VER >= 14