[Sema] Clean up extension binding a little

- Turn `BindExtensionsForIDEInspectionRequest` into the main extension
binding request.
- Change `ExtendedNominalRequest` such that it's no longer what
extension binding calls into to do the name lookup, instead it calls
directly into `computeExtendedNominal`. `getExtendedNominal` can
then be the entrypoint for `ExtendedNominalRequest` and assumes that
extension binding has already run. This avoids needing to fake the
dependency relationship in the DeclChecker.

lib/Parse/Parser.cpp

@@ -214,11 +214,7 @@ void Parser::performIDEInspectionSecondPassImpl(
 
   // Bind extensions if needed. This needs to be done here since we may have
   // mutated the AST above.
-  {
-    auto *M = SF->getParentModule();
-    BindExtensionsForIDEInspectionRequest req(M);
-    evaluateOrDefault(Context.evaluator, req, {});
-  }
+  bindExtensions(*SF->getParentModule());
 
   DoneParsingCallback->doneParsing(SF);