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

docs/UsingLibcxx.rst

@@ -173,3 +173,10 @@ thread safety annotations.
       return Tup{"hello world", 42}; // explicit constructor called. OK.
     }
 
+**_LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS**:
+  This macro disables the additional diagnostics generated by libc++ using the
+  `diagnose_if` attribute. These additional diagnostics include checks for:
+
+    * Giving `set`, `map`, `multiset`, `multimap` a comparator which is not
+      const callable.
+