Lack of cash is not one of Apple's problems. If they wanted to invest more in the US they could.

Most of the money will probably just sit still in a wide variety of investments (bonds and other stuff; Apple is one of the biggest players in these markets); the rest will go into dividends and share buybacks. A symbolic percentage might go into R&D and Trump will tweet about it.

(I might be wrong, but that is what I expect.)

They have R&D centers in Israel, China, Japan. Bringing the money back to the US is not necessary for this.

Most of this money is invested abroad already.

Or maybe QA, of which they seem to be lacking recently.