Merge #3706

3706: missing match arms diagnostic r=flodiebold a=JoshMcguigan

Following up on https://github.com/rust-analyzer/rust-analyzer/pull/3689#issuecomment-602718222, this PR creates a missing match arms diagnostic.

At the moment this is a very early draft, but I wanted to open it just to get some initial feedback.

Initial questions:

* Have I roughly created the correct boilerplate? 
* Inside the new `validate_match` function:
  * Am I correct in thinking I want to do validation by comparing the match arms against `match_expr`? And when analyzing `match_expr` I should be looking at it as a `hir_def::expr::Expr`?
  * I mostly copied the chained if-let statements from the struct validation. Shouldn't there be a non-failable way to get an AstPtr from the hir data structures? 

Thanks for all the guidance.

Co-authored-by: Josh Mcguigan <[email protected]>

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