Everywhere: Turn #if *_DEBUG into dbgln_if/if constexpr

Author: https://github.com/gunnarbeutner
Commit: 6cf59b6ae9
Pull-request: https://github.com/SerenityOS/serenity/pull/6796

Userland/Libraries/LibRegex/RegexLexer.cpp

@@ -130,9 +130,8 @@ Token Lexer::next()
         case '\\':
             return 2;
         default:
-#if REGEX_DEBUG
-            fprintf(stderr, "[LEXER] Found invalid escape sequence: \\%c (the parser will have to deal with this!)\n", peek(1));
-#endif
+            if constexpr (REGEX_DEBUG)
+                fprintf(stderr, "[LEXER] Found invalid escape sequence: \\%c (the parser will have to deal with this!)\n", peek(1));
             return 0;
         }
     };