[LLD] Improve the lit tests added by 272bd6f

The `cp` command will copy the permission bits from the original file to
the new one, which will cause permission denied (no written access) for the
following "echo" command in some system.

Switch to use `cat` which is more robust.

Author: hokein

lld/test/ELF/lto/devirt_validate_vtable_typeinfos.ll

@@ -3,7 +3,7 @@
 ;; Common artifacts
 ; RUN: opt --thinlto-bc -o %t1.o %s
 ; RUN: opt --thinlto-bc --thinlto-split-lto-unit -o %t1_hybrid.o %s
-; RUN: cp %s %t1_regular.ll
+; RUN: cat %s > %t1_regular.ll
 ; RUN: echo '!llvm.module.flags = !{!12, !13}' >> %t1_regular.ll
 ; RUN: echo '!12 = !{i32 1, !"ThinLTO", i32 0}' >> %t1_regular.ll
 ; RUN: echo '!13 = !{i32 1, !"EnableSplitLTOUnit", i32 1}' >> %t1_regular.ll

lld/test/ELF/lto/devirt_validate_vtable_typeinfos_no_rtti.ll

@@ -3,7 +3,7 @@
 ;; Common artifacts
 ; RUN: opt --thinlto-bc -o %t1.o %s
 ; RUN: opt --thinlto-bc --thinlto-split-lto-unit -o %t1_hybrid.o %s
-; RUN: cp %s %t1_regular.ll
+; RUN: cat %s > %t1_regular.ll
 ; RUN: echo '!llvm.module.flags = !{!6, !7}' >> %t1_regular.ll
 ; RUN: echo '!6 = !{i32 1, !"ThinLTO", i32 0}' >> %t1_regular.ll
 ; RUN: echo '!7 = !{i32 1, !"EnableSplitLTOUnit", i32 1}' >> %t1_regular.ll

lld/test/ELF/lto/devirt_validate_vtable_typeinfos_ref.ll

@@ -3,7 +3,7 @@
 ;; Common artifacts
 ; RUN: opt --thinlto-bc -o %t1.o %s
 ; RUN: opt --thinlto-bc --thinlto-split-lto-unit -o %t1_hybrid.o %s
-; RUN: cp %s %t1_regular.ll
+; RUN: cat %s > %t1_regular.ll
 ; RUN: echo '!llvm.module.flags = !{!2, !3}' >> %t1_regular.ll
 ; RUN: echo '!2 = !{i32 1, !"ThinLTO", i32 0}' >> %t1_regular.ll
 ; RUN: echo '!3 = !{i32 1, !"EnableSplitLTOUnit", i32 1}' >> %t1_regular.ll