Fix parsing of block expressions in "forbid_structs" contexts.

Forbidding block expressions entirely is too strict; instead, we should only
forbid them in contexts where we are parsing an optional RHS (i.e. the RHS of a
range expression).

crates/ra_syntax/src/grammar.ron

@@ -1,5 +1,5 @@
 // Stores definitions which must be used in multiple places
-// See `cargo gen-syntax` (defined in crates/tools/src/main.rs)
+// See `cargo gen-syntax` (defined in crates/ra_tools/src/main.rs)
 Grammar(
     punct: [
         (";", "SEMI"),