Thread safety analysis: Store CapabilityExprs in ScopedLockableFactEntry (NFC)

For now this doesn't make a whole lot of sense, but it will allow us to
store the capability kind in a CapabilityExpr and make sure it doesn't
get lost. The capabilities managed by a scoped lockable can of course be
of different kind, so we'll need to store that per entry.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D124128