Handle lookup into Builtin with module selector

Lookups like Builtin::Int64 were failing because BuiltinUnit rejected all unqualified lookups. Make it allow unqualified lookups with a module selector.

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -543,8 +543,8 @@ static void walkRelatedDecls(const ValueDecl *VD, const FnTy &Fn) {
     }
   } else {
     namelookup::lookupInModule(DC->getModuleScopeContext(),
-                               VD->getBaseName(), results,
-                               NLKind::UnqualifiedLookup,
+                               VD->getBaseName(), /*hasModuleSelector=*/false,
+                               results, NLKind::UnqualifiedLookup,
                                namelookup::ResolutionKind::Overloadable,
                                DC->getModuleScopeContext(),
                                VD->getLoc(),