Address feedback: update documentation, marka test unsupported in C++03.

Author: var-const

libcxx/docs/Hardening.rst

@@ -328,6 +328,20 @@ following options to the compiler:
 All the :ref:`same notes <notes-for-users>` apply to setting this macro as for
 setting ``_LIBCPP_HARDENING_MODE``.
 
+Notes for vendors
+-----------------
+
+Similarly to hardening modes, vendors can set the default assertion semantic by
+providing ``LIBCXX_ASSERTION_SEMANTIC`` as a configuration option, with the
+possible values of ``hardening_dependent``, ``ignore``, ``observe``,
+``quick_enforce`` and ``enforce``. The default value is ``hardening_dependent``
+which is a special value that instructs the library to select the semantic based
+on the hardening mode in effect (the mapping is described in
+:ref:`the main section on assertion semantics <assertion-semantics>`).
+
+This option controls both the assertion semantic that the precompiled library is
+built with and the default assertion semantic that users will build with.
+
 .. _override-assertion-handler:
 
 Overriding the assertion failure handler
@@ -447,6 +461,13 @@ The first character of an ABI tag encodes the hardening mode:
 - ``d`` -- [d]ebug mode;
 - ``n`` -- [n]one mode.
 
+The second character of an ABI tag encodes the assertion semantic:
+
+- ``i`` -- [i]gnore semantic;
+- ``o`` -- [o]bserve semantic;
+- ``q`` -- [q]uick-enforce semantic;
+- ``e`` -- [e]nforce semantic.
+
 Hardened containers status
 ==========================
 

libcxx/test/extensions/libcxx/odr_signature.assertion_semantics.sh.cpp

@@ -9,12 +9,12 @@
 // ABI tags have no effect in MSVC mode.
 // XFAIL: msvc
 
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
+// Assertion semantics are not supported in C++03 mode
+// UNSUPPORTED: std=c++03
 
 // Test that we encode the assertion semantic in an ABI tag to avoid ODR violations when linking TUs that have different
 // values for it.
 
-// Note that GCC doesn't support `-Wno-macro-redefined`.
 // RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU1  -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_IGNORE        -o %t.tu1.o
 // RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU2  -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_OBSERVE       -o %t.tu2.o
 // RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU3  -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE -o %t.tu3.o