Simplify calls to ast::Node::[to_]str()

Add helpers on Program and ProgramBuilder that significantly simplify
usage.
Also demangle - this also reduces a bunch of copy-pasta code.

Change-Id: I6215c346e7f6e49c20aced058a6150603253ed93
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/39342
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/type/test_helper.h

@@ -20,7 +20,6 @@
 #include <utility>
 
 #include "gtest/gtest.h"
-#include "src/demangler.h"
 #include "src/program_builder.h"
 
 namespace tint {
@@ -28,18 +27,7 @@ namespace type {
 
 /// Helper class for testing
 template <typename BASE>
-class TestHelperBase : public BASE, public ProgramBuilder {
- public:
-  /// Demangles the given string
-  /// @param s the string to demangle
-  /// @returns the demangled string
-  std::string demangle(const std::string& s) {
-    return demanger.Demangle(this, s);
-  }
-
-  /// A demangler
-  Demangler demanger;
-};
+class TestHelperBase : public BASE, public ProgramBuilder {};
 using TestHelper = TestHelperBase<testing::Test>;
 
 template <typename T>