[lit] Stop using PATH to lookup clang/lld/lldb unless requested

This patch stops lit from looking on the PATH for clang, lld and other
users of use_llvm_tool (currently only the debuginfo-tests) unless the
call explicitly requests to opt into using the PATH. When not opting in,
tests will only look in the build directory.

See the mailing list thread starting from
https://lists.llvm.org/pipermail/llvm-dev/2021-May/150421.html.

See the review for details of why decisions were made about when still
to use the PATH.

Reviewed by: thopre

Differential Revision: https://reviews.llvm.org/D102630