This patch adds feature modules registry, as discussed with @kadircet in
[discourse](https://discourse.llvm.org/t/rfc-registry-for-feature-modules/87733).
Feature modules, which added into the feature module set from registry
entries, can't expose public API, but still can be used via
`FeatureModule` interface.
As pointed out in D96244, "Module" is already pretty overloaded to refer
to clang and llvm modules. (And clangd deals directly with the former).
FeatureModule is a bit of a mouthful but it's pretty self-descriptive.
I think it might be better than "Component" which doesn't really capture
the "common interface" aspect - it's IMO confusing to refer to
"components" but exclude CDB for example.
Differential Revision: https://reviews.llvm.org/D97950
