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

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

This paves the way for other projects which might /use/ clang or
lld but not necessarily need to the full set of functionality
available to clang and lld tests to be able to have a basic set
of substitutions that allow a project to run the clang or lld
executables.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315627 91177308-0d34-0410-b5e6-96231b3b80d8