ast: Remove Node::set_source()

All nodes that don't have a Source constructor will need to have one added.
We can then find all missing Source mappings with a search for `Source{}`

Bug: tint:396
Bug: tint:390
Change-Id: I06f9689d4da0f3fd1bd757c7358dcc65f15dc752
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/35018
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/ast/node.h

@@ -47,9 +47,6 @@ class Node : public Castable<Node> {
 
   /// @returns the node source data
   const Source& source() const { return source_; }
-  /// Sets the source data
-  /// @param source the source data
-  void set_source(const Source& source) { source_ = source; }
 
   /// @returns true if the node is valid
   virtual bool IsValid() const = 0;

src/ast/test_helper.h

@@ -16,6 +16,7 @@
 #define SRC_AST_TEST_HELPER_H_
 
 #include <memory>
+#include <string>
 #include <utility>
 
 #include "gtest/gtest.h"