Autogen: Restore mocs_compilation in OBJECT libraries

Since commit f65f2093 (Autogen: Avoid processing CSharp targets,
2020-11-12, v3.20.0-rc1~301^2) we collect all sources for a target
earlier than previously.  Clear the sources cache so that it will be
re-computed later after AUTOGEN processing.

Fixes: #22085
32 jobs for !6024 with autogen-clear-early-source-cache in 77 minutes and 43 seconds (queued for 3 seconds)
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