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_var.cpp

@@ -74,7 +74,7 @@ auto HandleFieldDecl(Context& context) -> void {
     return;
   }
   context.PushState(state, StateKind::VarFinishAsField);
-  context.AddLeafNode(NodeKind::IdentifierNameNotBeforeParams, *identifier);
+  context.AddLeafNode(NodeKind::IdentifierNameNotBeforeSignature, *identifier);
   state.token = *colon;
   context.PushState(state, StateKind::VarAfterPatternAsField);
   context.PushState(StateKind::Expr);