Add a flag to make CHECK failures non-fatal for debugging. (#4835)

`toolchain/autoupdate_testdata.py --allow-check-fail` can now be used to
perform an autoupdate even if some `CARBON_CHECK`s are failing. What
this does will depend on how the toolchain behaves after the `CHECK`
failure, and of course there's no guarantees there, but this can be
useful if it's easier to debug the `CHECK` failure by looking at the
produced SemIR.

Internally, this uses `bazel build --config=non-fatal-checks`, which in
turn specifies a `--per_file_copt` for `check_internal.cpp`. The intent
here is that the rebuild required to enable or disable this mode is as
small as reasonably possible.

This mode is not compatible with `-c opt`, as it's important that check
failure calls are `[[noreturn]]` in `-c opt` mode.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

common/check_internal.cpp

@@ -9,11 +9,6 @@
 
 namespace Carbon::Internal {
 
-// Prints the buffered message.
-static auto PrintAfterStackTrace(void* str) -> void {
-  llvm::errs() << reinterpret_cast<char*>(str);
-}
-
 auto CheckFailImpl(const char* kind, const char* file, int line,
                    const char* condition_str, llvm::StringRef extra_message)
     -> void {
@@ -23,15 +18,29 @@ auto CheckFailImpl(const char* kind, const char* file, int line,
       llvm::StringRef(condition_str).empty() ? "" : ": ", condition_str,
       extra_message.empty() ? "" : ": ", extra_message);
 
+  // This macro is defined by `--config=non-fatal-checks`.
+#ifdef CARBON_NON_FATAL_CHECKS
+#ifdef NDEBUG
+#error "--config=non-fatal-checks is incompatible with -c opt"
+#endif
+  // TODO: It'd be nice to print the LLVM PrettyStackTrace, but LLVM doesn't
+  // expose functionality to do so.
+  llvm::sys::PrintStackTrace(llvm::errs());
+
+  llvm::errs() << message;
+#else
   // Register another signal handler to print the message. This is because we
   // want it at the bottom of output, after LLVM's builtin stack output, rather
   // than the top.
-  llvm::sys::AddSignalHandler(PrintAfterStackTrace,
-                              const_cast<char*>(message.c_str()));
+  llvm::sys::AddSignalHandler(
+      [](void* str) { llvm::errs() << reinterpret_cast<char*>(str); },
+      const_cast<char*>(message.c_str()));
+
   // It's useful to exit the program with `std::abort()` for integration with
   // debuggers and other tools. We also assume LLVM's exit handling is
   // installed, which will stack trace on `std::abort()`.
   std::abort();
+#endif
 }
 
 }  // namespace Carbon::Internal