Add InferCtxt::with_opaque_type_inference to get_body_with_borrowck_facts
`mir_borrowck` uses `with_opaque_type_inference` before calling `do_mir_borrowck`: 0eabf25b90/compiler/rustc_borrowck/src/lib.rs (L132)
However `get_body_with_borrowck_facts` does not. Therefore I get an ICE eg when calling this function on the bodies of an async function as described here: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/.E2.9C.94.20ICE.20when.20using.20get_body_with_borrowck_facts.20with.20async
This change fixes that bug.
r? `@nikomatsakis`