[funcattrs] Infer nosync from readnone and non-convergent

This implements the most basic possible nosync inference. The choice of inference rule is taken from the comments in attributor and the discussion on the review of the change which introduced the nosync attribute (06263672).

This is deliberately minimal. As noted in code comments, I do plan to add a more robust inference which actually scans the function IR directly, but a) I need to do some refactoring of the attributor code to use common interfaces, and b) I wanted to get something in. I also wanted to minimize the "interesting" analysis discussion since that's time intensive.

Context: This combines with existing nofree attribute inference to help prove dereferenceability in the ongoing deref-at-point semantics work.

Differential Revision: https://reviews.llvm.org/D99749