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/handle_array.cpp

@@ -5,6 +5,7 @@
 #include "toolchain/check/context.h"
 #include "toolchain/check/convert.h"
 #include "toolchain/check/handle.h"
+#include "toolchain/check/type.h"
 #include "toolchain/parse/node_kind.h"
 
 namespace Carbon::Check {
@@ -51,7 +52,7 @@ auto HandleParseNode(Context& context, Parse::ArrayExprId node_id) -> bool {
 
   bound_inst_id = ConvertToValueOfType(
       context, context.insts().GetLocId(bound_inst_id), bound_inst_id,
-      context.GetSingletonType(SemIR::IntLiteralType::SingletonInstId));
+      GetSingletonType(context, SemIR::IntLiteralType::SingletonInstId));
   context.AddInstAndPush<SemIR::ArrayType>(
       node_id, {.type_id = SemIR::TypeType::SingletonTypeId,
                 .bound_id = bound_inst_id,