[llvm-dev] Using clang/lld from the PATH when testing

[James Henderson via llvm-dev]

Hi all,

Summary: can we stop allowing clang/lld to be picked up from the PATH
environment when running testing?

Details:

As part of my work on a cross-project testsuite (see
https://reviews.llvm.org/D95339 and other related patches), I noticed that
where lit has been configured using `use_clang()` or `use_lld()` the
respective tools will be selected from the system PATH, if they aren't
found in the build directory. If users are running check-* to run the
relevant tests, this isn't an issue, as the tools will be built (and
therefore used in preference).

I personally, and I'm sure many others, routinely don't use these check-*
targets, preferring to manually build and run the subset of tests that have
the potential to be impacted by my change. Potentially this leaves room for
human error (e.g. a misconfigured build bot or simply a user error when
checking things) to not build clang (or lld) in a clean environment, and as
such, the "wrong" tool might be used, causing spurious tests failures (or
worse, spurious passes).

An environment variable ('CLANG') can be specified to specify the clang
executable to use, in preference to a built one or one on the PATH. I plan
to make a similar change for LLD soon too. As such, do we need the "use the
PATH version" behaviour anymore? Dropping it will allow simplifying code
and reduce room for human error.

Thanks,

James
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