Don't generate lldb inline test Makefiles if Makefile already exists.

Differential Revision: http://reviews.llvm.org/D6664
Reviewed by: Sean Callanan

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@224593 91177308-0d34-0410-b5e6-96231b3b80d8