Add and propagate template phase for constants. (#4964)

Treat template bindings as introducing template phase, and propagate it
in the same way we propagate the checked generic phase.

Rename "symbolic" to "checked symbolic" to make room for "template
symbolic". Also rename "phase" to "dependence".

toolchain/check/generic.cpp

@@ -11,6 +11,7 @@
 #include "toolchain/check/subst.h"
 #include "toolchain/check/type.h"
 #include "toolchain/check/type_completion.h"
+#include "toolchain/sem_ir/constant.h"
 #include "toolchain/sem_ir/generic.h"
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/inst.h"
@@ -46,12 +47,16 @@ auto StartGenericDefinition(Context& context) -> void {
 static auto AddGenericConstantInstToEvalBlock(
     Context& context, SemIR::GenericId generic_id,
     SemIR::GenericInstIndex::Region region, SemIR::InstId const_inst_id,
-    SemIR::InstId generic_inst_id) -> SemIR::ConstantId {
+    SemIR::InstId generic_inst_id, SemIR::ConstantDependence dependence)
+    -> SemIR::ConstantId {
   auto index = SemIR::GenericInstIndex(
       region, context.inst_block_stack().PeekCurrentBlockContents().size());
   context.inst_block_stack().AddInstId(generic_inst_id);
   return context.constant_values().AddSymbolicConstant(
-      {.inst_id = const_inst_id, .generic_id = generic_id, .index = index});
+      {.inst_id = const_inst_id,
+       .generic_id = generic_id,
+       .index = index,
+       .dependence = dependence});
 }
 
 namespace {
@@ -136,8 +141,11 @@ class RebuildGenericConstantInEvalBlockCallbacks final
   // constant.
   auto Rebuild(SemIR::InstId orig_inst_id, SemIR::Inst new_inst) const
       -> SemIR::InstId override {
-    auto const_inst_id =
-        context_.constant_values().GetConstantInstId(orig_inst_id);
+    auto& orig_symbolic_const = context_.constant_values().GetSymbolicConstant(
+        context_.constant_values().Get(orig_inst_id));
+    auto const_inst_id = orig_symbolic_const.inst_id;
+    auto dependence = orig_symbolic_const.dependence;
+
     // We might already have an instruction in the eval block if a transitive
     // operand of this instruction has the same constant value.
     auto result = constants_in_generic_.Insert(const_inst_id, [&] {
@@ -149,7 +157,7 @@ class RebuildGenericConstantInEvalBlockCallbacks final
       auto inst_id = context_.sem_ir().insts().AddInNoBlock(
           SemIR::LocIdAndInst::UncheckedLoc(loc_id_, new_inst));
       auto const_id = AddGenericConstantInstToEvalBlock(
-          context_, generic_id_, region_, const_inst_id, inst_id);
+          context_, generic_id_, region_, const_inst_id, inst_id, dependence);
       context_.constant_values().Set(inst_id, const_id);
       return inst_id;
     });