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.

llvm-svn: 291961
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: b1e7a12ee89ffcf50d0b18f8c6da7423604ea9d8
diff --git a/include/map b/include/map
index 9555aad..ecd9d92 100644
--- a/include/map
+++ b/include/map
@@ -453,9 +453,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Key, class _CP, class _Compare,
-          bool = is_empty<_Compare>::value && !__libcpp_is_final<_Compare>::value
-         >
+template <class _Key, class _CP, class _Compare, bool _IsSmall>
 class __map_value_compare
     : private _Compare
 {