Lit: Introduce an environment variable, $LIT_PRESERVES_TMP, to preserve TMP (and TEMP).

This is intended to check how many temporary files would be generated in automated builders.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192887 91177308-0d34-0410-b5e6-96231b3b80d8