[WGSL] GlobalVariableRewriter needs to visit variable initializers

https://bugs.webkit.org/show_bug.cgi?id=255515
rdar://108134621

Reviewed by Mike Wyrzykowski.

The global variable rewriter visitor overrides the Variable visitor, as it needs to record
variable definitions that may shadow globals, but it fails to visit the variable initializer
(if there is one), which means that globals used in the initializer are undetected. To fix
that we simply call the base visitor's visit method on the variable declaration.

* Source/WebGPU/WGSL/GlobalVariableRewriter.cpp:
(WGSL::RewriteGlobalVariables::visit):

Canonical link: https://commits.webkit.org/263062@main

Source/WebGPU/WGSL/GlobalVariableRewriter.cpp

@@ -118,6 +118,7 @@ void RewriteGlobalVariables::visit(AST::Function& function)
 void RewriteGlobalVariables::visit(AST::Variable& variable)
 {
     def(variable.name());
+    AST::Visitor::visit(variable);
 }
 
 void RewriteGlobalVariables::visit(AST::IdentifierExpression& identifier)