[torque] Use default constructor in place of self-delegation for Symbol()

This avoids a compile error when building with GCC in C++17 mode.

Bug: v8:12449
Change-Id: I14817895d31019fb71fc71b061f2ecf576dbc711
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3307102
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78171}

src/torque/earley-parser.h

@@ -248,7 +248,7 @@ class Rule final {
 // used in the parser.
 class Symbol {
  public:
-  Symbol() : Symbol({}) {}
+  Symbol() = default;
   Symbol(std::initializer_list<Rule> rules) { *this = rules; }
 
   // Disallow copying and moving to ensure Symbol has a stable address.