Rename findFunction from the ML side of the house to be named getFunction(),

making it more similar to the CFG side of things.  It is true that in a deeply
nested case that this is not a guaranteed O(1) time operation, and that 'get'
could lead compiler hackers to think this is cheap, but we need to merge these
and we can look into solutions for this in the future if it becomes a problem
in practice.

This is step 9/n towards merging instructions and statements, NFC.

PiperOrigin-RevId: 226983931