[clang-tidy] Use explicit default for constructor

See https://chromium.googlesource.com/chromium/src/+/HEAD/styleguide/c++/c++-dos-and-donts.md#prefer-to-use.

Bug: v8:10488
Change-Id: I3d2503b46172bc2fa310b24f04e944ff211ebf51
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2182310Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67626}

src/torque/ast.h

@@ -204,7 +204,7 @@ struct EnumDescription {
 
 class Ast {
  public:
-  Ast() {}
+  Ast() = default;
 
   std::vector<Declaration*>& declarations() { return declarations_; }
   const std::vector<Declaration*>& declarations() const {