Avoid parallelism problems generating lit.site.cfgs

extra/test/cpp11-migrate/Makefile was using the same tmp file for generating
lit.site.cfg for two different directories. Parallelism caused conflicts so now
using differently named temp files.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@176379 91177308-0d34-0410-b5e6-96231b3b80d8