[libc++] Remove _LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS (#90512)

I strongly suspect nobody ever used that macro since it wasn't very well
known. Furthermore, it only affects a handful of diagnostics and I think
it makes sense to either provide them unconditionally, or to not
provided them at all.

NOKEYCHECK=True
GitOrigin-RevId: a00bbcbe7c7b1d5fb437d530555a6940c0b8d06a

Author: ldionne

docs/ReleaseNotes/19.rst

@@ -118,6 +118,9 @@ Deprecations and Removals
   a ``std::basic_*fstream`` from a ``std::basic_string_view``, a input-iterator or a C-string, instead
   you can construct a temporary ``std::basic_string``. This change has been applied to C++17 and later.
 
+- The ``_LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS`` macro has been removed and is not honored anymore. Additional
+  warnings provided by libc++ as a matter of QoI will now be provided unconditionally.
+
 
 Upcoming Deprecations and Removals
 ----------------------------------

docs/UsingLibcxx.rst

@@ -167,15 +167,6 @@ safety annotations.
   build of libc++ which does not export any symbols, which can be useful when
   building statically for inclusion into another library.
 
-**_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` and their `unordered_`
-      counterparts a comparator which is not const callable.
-    * Giving an unordered associative container a hasher that is not const
-      callable.
-
 **_LIBCPP_NO_VCRUNTIME**:
   Microsoft's C and C++ headers are fairly entangled, and some of their C++
   headers are fairly hard to avoid. In particular, `vcruntime_new.h` gets pulled

include/__config

@@ -1390,7 +1390,7 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_NO_DESTROY
 #  endif
 
-#  if __has_attribute(__diagnose_if__) && !defined(_LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS)
+#  if __has_attribute(__diagnose_if__)
 #    define _LIBCPP_DIAGNOSE_WARNING(...) __attribute__((__diagnose_if__(__VA_ARGS__, "warning")))
 #  else
 #    define _LIBCPP_DIAGNOSE_WARNING(...)