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/handle_function.cpp

@@ -464,8 +464,7 @@ static auto BuildFunctionDecl(Context& context,
   SemIR::FunctionDecl function_decl = {SemIR::TypeId::None,
                                        SemIR::FunctionId::None,
                                        context.inst_block_stack().Pop()};
-  auto decl_id =
-      AddPlaceholderInst(context, SemIR::LocIdAndInst(node_id, function_decl));
+  auto decl_id = AddPlaceholderInst(context, node_id, function_decl);
   RequestVtableIfVirtual(context, node_id, virtual_modifier, parent_scope_inst,
                          decl_id);