transform: Fix PromoteInitializersToConstVar handling of for-loops

PromoteInitializersToConstVar was erroring on for loops that contained array or structure constructor expressions.

Added lots more tests.

Fixed: tint:1364
Change-Id: I033eaad94756ea496fc8bc5f03f39c6dba4e3a88
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/75580
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/sem/type_mappings.h

@@ -23,6 +23,7 @@ namespace tint {
 namespace ast {
 class CallExpression;
 class Expression;
+class ForLoopStatement;
 class Function;
 class MemberAccessorExpression;
 class Node;
@@ -39,6 +40,7 @@ namespace sem {
 class Array;
 class Call;
 class Expression;
+class ForLoopStatement;
 class Function;
 class MemberAccessorExpression;
 class Node;
@@ -56,6 +58,7 @@ struct TypeMappings {
   //! @cond Doxygen_Suppress
   Call* operator()(ast::CallExpression*);
   Expression* operator()(ast::Expression*);
+  ForLoopStatement* operator()(ast::ForLoopStatement*);
   Function* operator()(ast::Function*);
   MemberAccessorExpression* operator()(ast::MemberAccessorExpression*);
   Node* operator()(ast::Node*);