cmGeneratorTarget: Clear AllConfigSources in ClearSourcesCache

In commit 40aa6c05 (cmGeneratorTarget: Add method to collect all
sources for all configs, 2017-04-10, v3.9.0-rc1~268^2~5) we forgot to
update `ClearSourcesCache` to also clear `AllConfigSources`.  This leads
to subtle cases where code paths like PCH handling that add sources
during generation break depending on ordering.

Suggested-by: Christian Fersch
Fixes: #20712, #20702