Enables more aggressive metadata stripping by default. In this mode, the AOT compiler only generates reflection metadata for artifacts that were deemed reflection-visible by either dataflow analysis, or user inputs. Previously we would consider everything that got generated reflection-visible.

* Change the defaults within the compiler (making the conservative mode opt-in).
* Add tests for scenarios in https://github.com/dotnet/runtimelab/issues/656 (resolves dotnet/runtimelab#656).
* Update tests
* Do more marking where it was necessary