typeck aggregate rvalues in MIR type checker
This branch is an attempt to land content by @spastorino and @Nashenas88 that was initially landed on nll-master while we waited for https://github.com/rust-lang/rust/pull/45825 to land.
The biggest change it contains is that it extends the MIR type-checker to also type-check MIR aggregate rvalues (at least partially). Specifically, it checks that the operands provided for each field have the right type.
It does not yet check that their well-formedness predicates are met. That is https://github.com/rust-lang/rust/issues/45827. It also does not check other kinds of rvalues (that is https://github.com/rust-lang/rust/issues/45959). @spastorino is working on those issues now.
r? @arielb1