Suggestion: Control flow analysis

[danquirk]

This is a work item to track some of the ideas and work associated with implementing simple control flow analysis for error reporting purposes. @vladima has already implemented a version of this concept in #1287. It already covers some of the separate issues people have asked for but I'm adding a list of tasks here that can be checked off/added to by whoever ends up officially taking on this work.

• Use before definition Suggestion: disallow use before definition #21
• Switch case fall through Report errors for duplicate case clauses #854

[jbondc]

Can likely be used to improve type guards too, some pseudo-code here:
#1892 (comment)

Keep track of widened type assignments & when exiting a block, check if all the same & narrow to the same type.

[mhegazy]

Switch fall through is fixed by #4788

use before definition is tracked by #5207