check for is_cleanup violations in MIR typeck
check for is_cleanup violations in MIR typeck

There weren't any in practice, but as these cause MSVC-only problems, the
check looks like a good idea.

GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust