Adds cmGeneratorTarget::clearSourcesCache() which clears the cache
of precomputed sources lists and object names. The cache gets
recomputed on demand.

Clearing the cache is necessary in case an OBJECT library
gets a source added after a target (ORIGIN) that includes it
computed it's external sources cache.
This may happen in AUTOMOC when ORIGIN is processed before the
OBJECT library which gets a mocs_compilation.cpp file added.

Closes #17152
Closes #17139
Closes #17085
Closes #15967