Revert "[ast] Remove unused constructors and setters."

This reverts commit 4d28b2793512069dc23f88b2804f3594dbd80d45.

Reason for revert: Seeing weird build breakage ...

Original change's description:
> [ast] Remove unused constructors and setters.
>
> This CL removes unused default constructors and various set methods
> from the AST classes where they are not longer required.
>
> Change-Id: Ic437911c62d8c9e4354a1fa6bdc8483ce7511daf
> Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/34641
> Auto-Submit: dan sinclair <dsinclair@chromium.org>
> Reviewed-by: Ben Clayton <bclayton@google.com>
> Commit-Queue: dan sinclair <dsinclair@chromium.org>

TBR=dsinclair@chromium.org,bclayton@google.com

Change-Id: I9d5bf6fd6d47131650c964cad4e17a1cbe86b040
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/34682
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
diff --git a/src/ast/variable_decl_statement_test.cc b/src/ast/variable_decl_statement_test.cc
index daa7c2a..afd1538 100644
--- a/src/ast/variable_decl_statement_test.cc
+++ b/src/ast/variable_decl_statement_test.cc
@@ -43,10 +43,7 @@
 }
 
 TEST_F(VariableDeclStatementTest, IsVariableDecl) {
-  type::F32 f32;
-  auto* var = create<Variable>("a", StorageClass::kNone, &f32);
-
-  VariableDeclStatement s(var);
+  VariableDeclStatement s;
   EXPECT_TRUE(s.Is<VariableDeclStatement>());
 }
 
@@ -65,7 +62,7 @@
 }
 
 TEST_F(VariableDeclStatementTest, IsValid_NullVariable) {
-  VariableDeclStatement stmt(nullptr);
+  VariableDeclStatement stmt;
   EXPECT_FALSE(stmt.IsValid());
 }