Demangler: Change signature of primary Demangle() function

Have this take a SymbolTable instead of a Program.
Program will be split into Program (immutable) and ProgramBuilder (mutable). We'll need Demangler to support both.

Bug: tint:390
Change-Id: I6447dd9674919d4867ed8ba126880cdfd9bf7128
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38550
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/type/test_helper.h

@@ -35,7 +35,7 @@ class TestHelperBase : public BASE, public ast::BuilderWithProgram {
   /// @param s the string to demangle
   /// @returns the demangled string
   std::string demangle(const std::string& s) {
-    return demanger.Demangle(*mod, s);
+    return demanger.Demangle(mod->Symbols(), s);
   }
 
   /// A demangler