[include-cleaner] Make handling of enum constants similar to members

We were treating enum constants more like regular decls, which results
in ignoring type aliases/exports.
This patch brings the handling to be closer to member-like decls, with
one caveat. When we encounter reference to an enum constant we still
report an explicit reference to the particular enum constant, as
otherwise we might not see any references to the enum itself.

Also drops implicit references from qualified names to containers, as
we already have explicit references from the qualifier to relevant
container.

Differential Revision: https://reviews.llvm.org/D158515