Enhance parsing of CFG and Ext functions to optionally allow named arguments in

the function signature, giving them common functionality to ml functions.  This
is a strictly additive patch that adds new capability without changing behavior
in a significant way (other than a few diagnostic cleanups).  A subsequent
patch will change the printer to use this behavior, which will require updating
a ton of testcases.  :)

This exposes the fact that we need to make a grammar change for block
arguments, as is tracked by b/122119779

This is step 23/n towards merging instructions and statements, and one of the
first steps towards eliminating the "cfg vs ml" distinction at a syntax and
semantic level.

PiperOrigin-RevId: 227228342