Move default value inst ids to a dedicated value store. (#7810)

Per #7737 we move the default value `InstId` storage from
a block in the `SemIR::Function` data structure to a
`SemIR::File` scoped `ValueStore`.

Moves the default value consistency checking to the general
merge argument pattern matching logic, which changes the
error message issued to the generic one.

toolchain/check/pattern_match.cpp

@@ -962,21 +962,13 @@ auto MatchContext::DoPostWork(State state,
 
   // If a constant was specified, we should be able to convert it into the
   // type of the parameter.
-  auto raw_default_value_inst_id =
-      context_.full_pattern_stack()
-          .GetRawDefaultValues()[default_value_pattern.default_value_id.index];
-  auto converted_inst_id =
-      context_.insts().Is<SemIR::UnspecifiedValue>(raw_default_value_inst_id)
-          ? raw_default_value_inst_id
-          : ConvertToValueOfType(context_,
-                                 SemIR::LocId(raw_default_value_inst_id),
-                                 raw_default_value_inst_id, param_type_id);
-
-  // The index of the converted value should be the same as the raw value.
-  auto converted_default_id =
-      context_.full_pattern_stack().AddConvertedDefaultValue(converted_inst_id);
-  CARBON_CHECK(converted_default_id.index ==
-               default_value_pattern.default_value_id.index);
+  auto& default_value =
+      context_.default_values().Get(default_value_pattern.default_value_id);
+  if (!default_value.is_unspecified) {
+    default_value.value_id =
+        ConvertToValueOfType(context_, SemIR::LocId(default_value.raw_id),
+                             default_value.raw_id, param_type_id);
+  }
 
   results_stack_.PopArray();