Add support for default values for non-generic parameters of generic functions. (#8011)

A first step in expanding support for default values for parameters in
generic functions.

Adds code to parameter type deduction to include default values for
absent call arguments. In support of that, this PR also generalizes a
function from `export.cpp` that is useful to extract a default
`ConstantId` value from an arbitrary function paramater pattern.

toolchain/check/pattern.cpp

@@ -360,4 +360,26 @@ auto PerformAction(Context& context, SemIR::LocId /*loc_id*/,
   }
 }
 
+auto ExtractParameterDefault(Context& context,
+                             SemIR::InstId param_pattern_inst_id)
+    -> SemIR::ConstantId {
+  auto pattern_id =
+      context.constant_values().GetConstantInstId(param_pattern_inst_id);
+  while (pattern_id.has_value()) {
+    CARBON_KIND_SWITCH(context.insts().Get(pattern_id)) {
+      case CARBON_KIND(SemIR::DefaultValuePattern inst): {
+        return context.constant_values().Get(inst.value_id);
+      }
+      case CARBON_KIND_ANY(SemIR::AnyBindingPattern, inst): {
+        pattern_id = inst.subpattern_id;
+        break;
+      }
+      default: {
+        return SemIR::ConstantId::None;
+      }
+    }
+  }
+  return SemIR::ConstantId::None;
+}
+
 }  // namespace Carbon::Check