[SourceKit/CursorInfo] Add constructor to call results

Cursor info for a constructor would previously give the cursor info for
the containing type only. It now also adds cursor info for the
constructor itself in a "secondary_symbols" field.

Refactor `passCursorInfoForDecl` to use a single allocator rather than
keeping track of positions in a buffer and assigning everything at the
end of the function.

Refactor the various available refactoring gathering functions to take a
SmallVectorImpl and to not copy strings where they don't need to.

Resolves rdar://75385556

lib/Serialization/ModuleFile.cpp

@@ -1157,7 +1157,7 @@ ModuleFile::getSourceOrderForDecl(const Decl *D) const {
   return Triple.getValue().SourceOrder;
 }
 
-void ModuleFile::collectAllGroups(std::vector<StringRef> &Names) const {
+void ModuleFile::collectAllGroups(SmallVectorImpl<StringRef> &Names) const {
   if (!Core->GroupNamesMap)
     return;
   for (auto It = Core->GroupNamesMap->begin(); It != Core->GroupNamesMap->end();