Refactor AddInst-family functions to their own file (#4941)

This in particular uses free functions because it's likely to end up
more consistent with types (versus a wrapper object for InstStore).
Note, this is unlikely to have a performance impact, but if it does, we
can look into related approaches (and we've already discussed using
LTO).

Renames `PendingBlock::AddInst` to `PendingBlock::Add` because
`MakeElementAccessInst` expects the matching name to exist.

toolchain/check/interface.cpp

@@ -7,6 +7,7 @@
 #include "toolchain/check/context.h"
 #include "toolchain/check/eval.h"
 #include "toolchain/check/generic.h"
+#include "toolchain/check/inst.h"
 #include "toolchain/check/type.h"
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/inst.h"
@@ -36,8 +37,8 @@ auto BuildAssociatedEntity(Context& context, SemIR::InterfaceId interface_id,
   // Name lookup for the declaration's name should name the associated entity,
   // not the declaration itself.
   auto type_id = GetAssociatedEntityType(context, self_type_id);
-  return context.AddInst<SemIR::AssociatedEntity>(
-      context.insts().GetLocId(decl_id),
+  return AddInst<SemIR::AssociatedEntity>(
+      context, context.insts().GetLocId(decl_id),
       {.type_id = type_id, .index = index, .decl_id = decl_id});
 }