add include-what-you-use to clang build

[knoepfel]

This issue has been migrated from https://cdcvs.fnal.gov/redmine/issues/24109 (FNAL account required)
Originally created by @rlcee on 2020-03-02 18:02:32

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We have started exploring using include-what-you-use to
clean up our code base. We are building it in the out-of-tree
method, which is awkward. Please consider integrating this as
part of your clang build. If you can accept, then of course
we would like to see a new clang build soon. Thanks.

[knoepfel]

Comment by @chissg on 2020-03-09 15:26:34

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This is certainly a reasonable request. We have started looking into a Clang 9 build but have encountered issues producing a bootstrapped standalone build. It might take a little while, but this is in the plan.