Reduce explicit SemIR::LocIdAndInst construction (#5153)

Building on #5151 reducing `UncheckedLoc` use, further remove uses of
the `SemIR::LocIdAndInst` constructor where we typically have overloads
that don't need it. Add parallel convenience wrappers for placeholder
insts.

Also refactors `MergeReplacing`. I don't think it makes sense to add an
overload for `ReplaceLocIdAndInstBeforeConstantUse`, but we can still
reduce the `LocIdAndInst` construction there.

toolchain/check/import_cpp.cpp

@@ -226,8 +226,8 @@ static auto ImportFunctionDecl(Context& context, SemIR::LocId loc_id,
 
   auto function_decl = SemIR::FunctionDecl{
       SemIR::TypeId::None, SemIR::FunctionId::None, SemIR::InstBlockId::Empty};
-  auto decl_id = AddPlaceholderInst(
-      context, SemIR::LocIdAndInst(Parse::NodeId::None, function_decl));
+  auto decl_id =
+      AddPlaceholderInst(context, Parse::NodeId::None, function_decl);
 
   auto function_info = SemIR::Function{
       {.name_id = name_id,