build herwig7 and rthepeg in debug mode for CLANG IBs

[smuzaffar]

No description provided.

[cmsbuild]

A new Pull Request was created by @smuzaffar for branch master.

@akritkbehera, @cmsbuild, @iarspider, @raoatifshad, @smuzaffar can you please review it and eventually sign? Thanks.
@ftenchini, @mandrenguyen, @sextonkennedy you are the release manager for this.
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[cmsbuild]

cms-bot internal usage

[cmsbuild]

REMINDER @ftenchini, @mandrenguyen, @sextonkennedy: This PR was tested with cms-sw/cmsdist#10410, please check if they should be merged together

[smuzaffar]

+externals

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next master IBs after it passes the integration tests. This pull request will now be reviewed by the release team before it's merged. @mandrenguyen, @ftenchini, @sextonkennedy (and backports should be raised in the release meeting by the corresponding L2)