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_requirement.cpp

@@ -18,7 +18,8 @@ auto HandleRequirementBegin(Context& context) -> void {
       context.PositionKind(static_cast<Lookahead>(2)) ==
           Lex::TokenKind::Equal) {
     auto period = context.Consume();
-    context.AddNode(NodeKind::IdentifierNameNotBeforeParams, context.Consume(),
+    context.AddNode(NodeKind::IdentifierNameNotBeforeSignature,
+                    context.Consume(),
                     /*has_error=*/false);
     context.AddNode(NodeKind::DesignatorExpr, period, /*has_error=*/false);
     state.token = context.Consume();