Revert "[lit] Raise the logic for enabling clang & lld substitutions to llvm."

This reverts commit r315627, fixing bot failures:
http://green.lab.llvm.org/green/job/clang-stage1-configure-RA

LIT is failing to properly apply substitution to debuginfo-tests
after this change.

rdar://problem/34979568

llvm-svn: 315712