In clang-format, OpenCL .cl file uses default C++ formatting. There are
many pull-requests in libclc project that change OpenCL files. It is
beneficial to enable clang-format for them in CI.

Only check for diffs containing "undef" in .ll files, this prevents
comments like `// We should not have undef values...` triggering the
undef checker bot.

If the list of filtered files was empty the check would process every
file in the diff.

This patch skips checking files with a .mir extension for the presence
of undef. This was creating false positives that got reported on
discourse.

As discussed in
https://discourse.llvm.org/t/please-dont-use-undef-in-tests-part-2/83388,
this patch adds a comment to PRs that introduce new uses of undef.

It uses the the `git diff -S' command to find new uses, avoiding warning
about old uses. It further trims down with a regex to get only added (+)
lines.

The default mode does not provide a way to see the actual failure of the
formatters without modifying the code. Instead offer the user the option
to rerun with a `FORMAT_HOOK_VERBOSE=1` environment variable to print
the actual formatting diff.

This patch ensures that the CI script controls which file extensions are
considered instead of letting git-clang-format apply its own filtering
rules. In particular, this properly handles libc++ extension-less
headers which were passed to git-clang-format, but then dropped by 
the tool as having an unrecognized extension.

This is a second attempt to land 7620fe0d2d1e, which was reverted in
9572388 because it caused formatting not to be enforced for several patches.
The problem was that we'd incorrectly pass the extensions with additional
quoting to git-clang-format. The incorrect quoting has been removed in this
version of the patch.