[Parse][CodeCompletion] Completions for precedencegroup decls

Added the 'Module::getPrecedenceGroups' API to separate precedence group lookup
from 'Module::lookupVisibleDecls', which together with 'FileUnit::lookupVisibleDecls',
to which the former is forwarded, are expected to look up only 'ValueDecl'. In particular, this
prevents completions like Module.PrecedenceGroup.

include/swift/IDE/CodeCompletionCache.h

@@ -95,6 +95,7 @@ struct RequestedCachedModule {
   CodeCompletionCache::Key Key;
   const ModuleDecl *TheModule;
   bool OnlyTypes;
+  bool OnlyPrecedenceGroups;
 };
 
 } // end namespace ide