[libc++][hardening] Add a greppable prefix to assertion messages.

The current assertion failure messages produced by Hardening are not
very grep-friendly (the common part is rarther generic and requires
wildcards to match). While it's possible to use `__FILE__` for grepping,
it's easier and more straighforward to simply add a libc++-specific
prefix; this is especially important for the planned `observe` mode that
might produce many assertion failure messages over the course of the
program's execution that later need to be filtered and examined.

Author: var-const

libcxx/include/__assert

@@ -20,8 +20,8 @@
 #define _LIBCPP_ASSERT(expression, message)                                                                            \
   (__builtin_expect(static_cast<bool>(expression), 1)                                                                  \
        ? (void)0                                                                                                       \
-       : _LIBCPP_ASSERTION_HANDLER(__FILE__ ":" _LIBCPP_TOSTRING(__LINE__) ": assertion " _LIBCPP_TOSTRING(            \
-             expression) " failed: " message "\n"))
+       : _LIBCPP_ASSERTION_HANDLER(__FILE__ ":" _LIBCPP_TOSTRING(                                                      \
+             __LINE__) ": libc++ Hardening assertion " _LIBCPP_TOSTRING(expression) " failed: " message "\n"))
 
 // WARNING: __builtin_assume can currently inhibit optimizations. Only add assumptions with a clear
 // optimization intent. See https://discourse.llvm.org/t/llvm-assume-blocks-optimization/71609 for a

libcxx/test/support/check_assertion.h

@@ -47,7 +47,7 @@ using Matcher     = std::function<MatchResult(const std::string& /*text*/)>;
 MatchResult MatchAssertionMessage(const std::string& text, std::string_view expected_message) {
   // Extract information from the error message. This has to stay synchronized with how we format assertions in the
   // library.
-  std::regex assertion_format(".*###\\n(.*):(\\d+): assertion (.*) failed: (.*)\\n###");
+  std::regex assertion_format(".*###\\n(.*):(\\d+): libc\\+\\+ Hardening assertion (.*) failed: (.*)\\n###");
 
   std::smatch match_result;
   bool has_match = std::regex_match(text, match_result, assertion_format);