don't hard override post_install_step in Clang easyblock, but extend it by calling out to parent easyblock first

[boegel]

post_install_step was redefined in #2721 by @mboisson to support installing Clang with Python bindings, but the default implementation of post_install_step in EasyBlock does a bunch of (potentially) useful things that we should keep doing...

See also https://github.com/easybuilders/easybuild-framework/blob/4c3b0522f6d2df50078a563f0161ba18b767851f/easybuild/framework/easyblock.py#L2861

[branfosj]

lgtm

[branfosj]

Test report by @branfosj

Overview of tested easyconfigs (in order)

• SUCCESS Clang-13.0.1-GCCcore-11.2.0.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
bear-pg0211u03a.bear.cluster - Linux RHEL 8.6, x86_64, Intel(R) Xeon(R) Gold 6248 CPU @ 2.50GHz (cascadelake), Python 3.6.8
See https://gist.github.com/444253ad3bdd91b97049b153f0cb30ac for a full test report.

[branfosj]

Going in, thanks @boegel!

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS Clang-9.0.1-GCCcore-9.3.0.eb
• SUCCESS Clang-12.0.1-GCCcore-10.3.0.eb
• SUCCESS Clang-13.0.1-GCCcore-11.2.0.eb

Build succeeded for 3 out of 3 (3 easyconfigs in total)
node3141.skitty.os - Linux RHEL 8.4, x86_64, Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz, Python 3.6.8
See https://gist.github.com/5d06130aff5842f0f4eed31060aae0c3 for a full test report.