Fix handling of const patterns

E.g. in `match x { None => ... }`, `None` is a path pattern (resolving to the
option variant), not a binding. To determine this, we need to try to resolve the
name during lowering. This isn't too hard since we already need to resolve names
for macro expansion anyway (though maybe a bit hacky).

Fixes #1618.

crates/ra_hir_def/src/expr.rs

@@ -48,7 +48,7 @@ pub enum Literal {
 
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub enum Expr {
-    /// This is produced if syntax tree does not have a required expression piece.
+    /// This is produced if the syntax tree does not have a required expression piece.
     Missing,
     Path(Path),
     If {