Track the start of a signature more accurately (#6760)

This change ensures that a function signature always starts with an
`IdentifierNameMaybeBeforeSignature` node (renamed from
`IdentifierNameBeforeParams`), even in the case of function declarations
like `fn F -> T` that have no parameter list. As a consequence, this
ensures that we push new entries onto `pattern_block_stack` and
`full_pattern_stack` when we start processing the function signature.

toolchain/parse/handle_choice.cpp

@@ -67,10 +67,10 @@ auto HandleChoiceAlternative(Context& context) -> void {
   }
 
   if (context.PositionIs(Lex::TokenKind::OpenParen)) {
-    context.AddLeafNode(NodeKind::IdentifierNameBeforeParams, *token);
+    context.AddLeafNode(NodeKind::IdentifierNameMaybeBeforeSignature, *token);
     context.PushState(StateKind::PatternListAsExplicit);
   } else {
-    context.AddLeafNode(NodeKind::IdentifierNameNotBeforeParams, *token);
+    context.AddLeafNode(NodeKind::IdentifierNameNotBeforeSignature, *token);
   }
 }