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/node_ids.h

@@ -186,7 +186,7 @@ using AnyPackagingDeclId =
 using AnyPointerDeferenceExprId =
     NodeIdOneOf<PrefixOperatorStarId, PointerMemberAccessExprId>;
 using AnyRuntimeBindingPatternName =
-    NodeIdOneOf<IdentifierNameNotBeforeParamsId, SelfValueNameId,
+    NodeIdOneOf<IdentifierNameNotBeforeSignatureId, SelfValueNameId,
                 UnderscoreNameId>;
 using AnyPrimitiveFormIdId =
     NodeIdOneOf<RefPrimitiveFormId, VarPrimitiveFormId, ValPrimitiveFormId>;