[flang] Feature list plugin

Plugin that counts the number of times each tree node occurs in a given program.  Used for test coverage.

> One thing we need...is a way to determine what features a code uses. Preferably we would also be able to determine if they are implemented or not. Just the former could be done with a visitor for the parse tree. For the latter we would continue compilation and somehow ignore todo errors but collect them - @jdoerfert

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D143704