Binding pattern naming cleanup (#3410)

- Rename `PatternBinding` to `BindingPattern`.
- Use `BindingPattern` rather than `Pattern` in the names of
binding-pattern-specific parse states.

toolchain/parse/handle_let.cpp

@@ -16,7 +16,7 @@ auto HandleLet(Context& context) -> void {
   context.AddLeafNode(NodeKind::LetIntroducer, context.Consume());
 
   // This will start at the pattern.
-  context.PushState(State::PatternAsLet);
+  context.PushState(State::BindingPatternAsLet);
 }
 
 auto HandleLetAfterPattern(Context& context) -> void {