Fix canonicalization and simplify default value patterns. (#7837)

Removes support for unspecified default values. Fixes canonicalization
of the `DefaultValuePattern` instruction by making them immutable after
they are issued, and by removing the `DefaultValueId` operand which
wasn't being canonicalized.

toolchain/parse/handle_pattern_list.cpp

@@ -190,7 +190,6 @@ auto HandlePatternListElementCheckForDefaultValue(Context& context) -> void {
     return;
   }
 
-  // No underscore, we parse this as a normal expression.
   context.PushStateForExpr(state.ambient_precedence);
 }