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/utilities/tuple/tuple.tuple/diagnose_reference_binding.fail.cpp

@@ -30,4 +30,11 @@ int main() {
     // bind rvalue to constructed non-rvalue
     std::tuple<std::string &&> t2("hello"); // expected-note {{requested here}}
     std::tuple<std::string &&> t3(std::allocator_arg, alloc, "hello"); // expected-note {{requested here}}
+
+    // FIXME: The below warnings may get emitted as an error, a warning, or not emitted at all
+    // depending on the flags used to compile this test.
+  {
+    // expected-warning@tuple:* 0+ {{binding reference member 'value' to a temporary value}}
+    // expected-error@tuple:* 0+ {{binding reference member 'value' to a temporary value}}
+  }
 }