[ThinLTO] Modify test to help diagnose bot failures

I am getting bot failures from r335760 that are difficult to diagnose
since the stderr is getting redirected to FileCheck. Save and dump the
debug output to stderr to help debug the issue.

llvm-svn: 335764
diff --git a/llvm/test/ThinLTO/X86/deadstrip.ll b/llvm/test/ThinLTO/X86/deadstrip.ll
index 2860d52..69a99fc 100644
--- a/llvm/test/ThinLTO/X86/deadstrip.ll
+++ b/llvm/test/ThinLTO/X86/deadstrip.ll
@@ -23,7 +23,9 @@
 ; RUN:   -r %t2.bc,_dead_func,l \
 ; RUN:   -r %t2.bc,_another_dead_func,pl \
 ; RUN:   -thinlto-threads=1 \
-; RUN:	 -debug-only=function-import 2>&1 | FileCheck %s --check-prefix=DEBUG
+; RUN:	 -debug-only=function-import >%t2.out.debug 2>&1
+; RUN: cat %t2.out.debug
+; RUN: cat %t2.out.debug | FileCheck %s --check-prefix=DEBUG
 ; RUN: llvm-dis < %t.out.1.3.import.bc | FileCheck %s --check-prefix=LTO2
 ; RUN: llvm-dis < %t.out.2.3.import.bc | FileCheck %s --check-prefix=LTO2-CHECK2
 ; RUN: llvm-nm %t.out.1 | FileCheck %s --check-prefix=CHECK2-NM