[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/SerializedModuleLoader.cpp

@@ -1264,8 +1264,8 @@ SerializedASTFile::getSourceOrderForDecl(const Decl *D) const {
   return File.getSourceOrderForDecl(D);
 }
 
-void
-SerializedASTFile::collectAllGroups(std::vector<StringRef> &Names) const {
+void SerializedASTFile::collectAllGroups(
+    SmallVectorImpl<StringRef> &Names) const {
   File.collectAllGroups(Names);
 };