[lld/mac] Add test for --lto-O

This belongs to fe08e9c, I (thakis) forgot to `git add` it back then.

Differential Revision: https://reviews.llvm.org/D105223

lld/test/MachO/lto-opt-level.ll

@@ -0,0 +1,28 @@
+; REQUIRES: x86
+
+; RUN: rm -rf %t; mkdir %t
+; RUN: llvm-as  %s -o %t/test.o
+
+; RUN: %lld %t/test.o --lto-O0 -o %t/test
+; RUN: llvm-nm -pa %t/test | FileCheck %s --check-prefixes=CHECK-O0
+
+; RUN: %lld %t/test.o --lto-O2 -o %t/test
+; RUN: llvm-nm -pa %t/test | FileCheck %s --check-prefixes=CHECK-O2
+
+; RUN: %lld %t/test.o -o %t/test
+; RUN: llvm-nm -pa %t/test | FileCheck %s --check-prefixes=CHECK-O2
+
+; CHECK-O0: foo
+; CHECK-O2-NOT: foo
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define internal void @foo() {
+  ret void
+}
+
+define void @main() {
+  call void @foo()
+  ret void
+}