Sync upstream 2020 05 07

[vext01]

This is a second attempt at #102

This syncs upstream as of 2020-05-07.

Most of the things I had to fix were uninteresting apart from the mir-opt test change.

Our SIR tests used to piggy back on a lot of mir-opt test infrastructure, but they've since changed all of that and removed the ability to elide in expected test outcomes. We have tests that use elision for good reason, so I've forward ported some of the old mir-opt test stuff into our tree.

That stuff is in runtest_sir.rs.

Tests are passing, and I also checked yk works too.

[vext01]

don't give me trouble now bors.

bors ping

[bors[bot]]

pong

[vext01]

bors try

[vext01]

bors try

[bors[bot]]

try

Already running a review

[vext01]

bors try-

[vext01]

bors try

[vext01]

bors try-

[vext01]

@ptersilie looks like this may work now!

[ltratt]

bors r+

[bors[bot]]

Build succeeded:

• buildbot/ykrustc-buildbot-build-script-hwt