Bug 1585234 - Part 1: Remove unnecessary Variant wrap/match on context. r=Yoric

A half of Parser methods are using single context type, and remaining are using
2 context types.  There are no place that uses 3 context types necessarily.

Part 1 modifies the code generator to collect context type information, and
reflect the information to Parser methods, to use single non-Variant type
if possible.

Also changed corresponding TokenReader methods to use single non-Variant type.

Differential Revision: https://phabricator.services.mozilla.com/D48011

js/src/frontend/BinASTParserPerTokenizer.cpp

@@ -134,7 +134,7 @@ JS::Result<ParseNode*> BinASTParserPerTokenizer<Tok>::parseAux(
   MOZ_TRY(tokenizer_->readHeader());
 
   ParseNode* result(nullptr);
-  const Context topContext((RootContext()));
+  const auto topContext = RootContext();
   MOZ_TRY_VAR(result, asFinalParser()->parseProgram(topContext));
 
   mozilla::Maybe<GlobalScope::Data*> bindings =
@@ -197,7 +197,7 @@ JS::Result<FunctionNode*> BinASTParserPerTokenizer<Tok>::parseLazyFunction(
 
   // Inject a toplevel context (i.e. no parent) to parse the lazy content.
   // In the future, we may move this to a more specific context.
-  const Context context((RootContext()));
+  const auto context = Context(RootContext());
   MOZ_TRY(
       (asFinalParser()->*parseFunc)(func->nargs(), &params, &tmpBody, context));