Add a flag to dump the C++ AST (#5918)

Use it to dump the AST that includes a generated C++ thunk.
Based on #5917.

Also added printing of the full actual text when check fails to make
debugging easier.
Changed line replacement to allow removing complete lines.

Part of #5514.

testing/file_test/autoupdate.cpp

@@ -201,6 +201,9 @@ auto FileTestAutoupdater::BuildCheckLines(llvm::StringRef output,
     }
 
     do_extra_check_replacements_(check_line);
+    if (check_line.empty()) {
+      continue;
+    }
 
     if (default_file_re_) {
       absl::string_view filename;