Move type functions off Context (#4951)

This creates a new check/type.h for most logic, and also moves some
functions to TypeStore in sem_ir/type.h. My approach for TypeStore is to
focus on moving the read-only functions there.

toolchain/check/call.cpp

@@ -9,6 +9,7 @@
 #include "toolchain/check/convert.h"
 #include "toolchain/check/deduce.h"
 #include "toolchain/check/function.h"
+#include "toolchain/check/type.h"
 #include "toolchain/diagnostics/format_providers.h"
 #include "toolchain/sem_ir/builtin_function_kind.h"
 #include "toolchain/sem_ir/entity_with_params_base.h"
@@ -118,8 +119,9 @@ static auto PerformCallToGenericInterface(
   if (!callee_specific_id) {
     return SemIR::ErrorInst::SingletonInstId;
   }
-  return context.GetOrAddInst(loc_id, context.FacetTypeFromInterface(
-                                          interface_id, *callee_specific_id));
+  return context.GetOrAddInst(
+      loc_id,
+      FacetTypeFromInterface(context, interface_id, *callee_specific_id));
 }
 
 auto PerformCall(Context& context, SemIR::LocId loc_id, SemIR::InstId callee_id,
@@ -164,8 +166,8 @@ auto PerformCall(Context& context, SemIR::LocId loc_id, SemIR::InstId callee_id,
     callee_id = context.GetOrAddInst(
         context.insts().GetLocId(callee_id),
         SemIR::SpecificFunction{
-            .type_id = context.GetSingletonType(
-                SemIR::SpecificFunctionType::SingletonInstId),
+            .type_id = GetSingletonType(
+                context, SemIR::SpecificFunctionType::SingletonInstId),
             .callee_id = callee_id,
             .specific_id = *callee_specific_id});
     if (callee_function.self_type_id.has_value()) {
@@ -201,7 +203,7 @@ auto PerformCall(Context& context, SemIR::LocId loc_id, SemIR::InstId callee_id,
       // For functions with an implicit return type, the return type is the
       // empty tuple type.
       if (!return_info.type_id.has_value()) {
-        return_info.type_id = context.GetTupleType({});
+        return_info.type_id = GetTupleType(context, {});
       }
       break;
     case SemIR::InitRepr::ByCopy: