Move let expression checking to parsing

There was an incomplete version of the check in parsing and a second
version in AST validation. This meant that some, but not all, invalid
uses were allowed inside macros/disabled cfgs. It also means that later
passes have a hard time knowing when the let expression is in a valid
location, sometimes causing ICEs.

- Add a field to ExprKind::Let in AST/HIR to mark whether it's in a
  valid location.
- Suppress later errors and MIR construction for invalid let
  expressions.

compiler/rustc_hir_analysis/src/check/region.rs

@@ -149,7 +149,7 @@ fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx h
                     // From now on, we continue normally.
                     visitor.cx = prev_cx;
                 }
-                hir::StmtKind::Local(..)=> {
+                hir::StmtKind::Local(..) => {
                     // Each declaration introduces a subscope for bindings
                     // introduced by the declaration; this subscope covers a
                     // suffix of the block. Each subscope in a block has the