MIR-borrowck: Add support for `union` types

[pnkfelix]

The MIR-borrowck prototype code left out support for union. Luckily we have test cases that illustrate the omission.

• You can see list of such tests in the discrepancy spreadsheet.
• But for ease of reference, I am also transcribing that list into the first comment on this issue.

So the next step is to incorporate support for union into librustc_mir/borrow_check.rs.

[pnkfelix]

Transcription of relevant tests according to the discrepancy spreadsheet. (caveat: data is based on out-of-date pnkfelix/mir-borrowck4 branch)

tests failing because union support is missing from MIR-borrowck ...
borrowck-union-borrow-nested.rs
borrowck-union-borrow.rs
borrowck-union-uninitialized.rs

[KiChjang]

I'm interested in tackling this.

[arielb1]

@KiChjang

So I think a useful first step would be to integrate the spec for borrowing unions here:
https://github.com/rust-lang/rfcs/blob/master/text/1444-union.md#borrowing-union-fields

into the main borrowck spec (yes I know it's called "nll", but it also applies to MIR borrowck) - preferably as a PR against rust-lang/rfcs:
https://github.com/rust-lang/rfcs/blob/master/text/2094-nll.md#borrow-checker-phase-2-reporting-errors

From that, we should be able to see what changes need to be done to the borrow-checker implementation and do them.

[KiChjang]

PR to make changes to the RFC: rust-lang/rfcs#2174