Merge #1562

1562: Continue support for .await r=matklad a=unrealhoang

- add await expr to ast and HIR Expr
- infer type for `.await`

Co-authored-by: Unreal Hoang <unrealhoang@gmail.com>

diff --cc crates/ra_hir/src/expr.rs
Simple merge

diff --cc crates/ra_hir/src/ty/tests.rs
Simple merge