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