Add _LIBCPP_DIAGNOSE_WARNING and _LIBCPP_DIAGNOSE_ERROR macros.

Clang recently added a `diagnose_if(cond, msg, type)` attribute
which can be used to generate diagnostics when `cond` is a constant
expression that evaluates to true. Otherwise no attribute has no
effect.

This patch adds _LIBCPP_DIAGNOSE_ERROR/WARNING macros which
use this new attribute. Additionally this patch implements
a diagnostic message when a non-const-callable comparator is
given to a container.

Note: For now the warning version of the diagnostic is useless
within libc++ since warning diagnostics are suppressed by the
system header pragma. I'm going to work on fixing this.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291961 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/compiler.py

@@ -296,7 +296,7 @@ class CXXCompiler(object):
 
     def addWarningFlagIfSupported(self, flag):
         if self.hasWarningFlag(flag):
-            assert flag not in self.warning_flags
-            self.warning_flags += [flag]
+            if flag not in self.warning_flags:
+                self.warning_flags += [flag]
             return True
         return False