[Completion] Only provide macro completions when they are valid

Only return macros that are valid in their current position, ie. an
attached macro is not valid on a nominal.

Also return freestanding expression macros in code block item position
and handle the new freestanding code item macros.

Resolves rdar://105563583.

include/swift/IDE/CodeCompletionCache.h

@@ -116,9 +116,7 @@ public:
 struct RequestedCachedModule {
   CodeCompletionCache::Key Key;
   const ModuleDecl *TheModule;
-  bool OnlyTypes;
-  bool OnlyPrecedenceGroups;
-  bool OnlyMacros;
+  CodeCompletionFilter Filter;
 };
 
 } // end namespace ide