Add an `ilverify` flag for verifying `UnsafeAccessor`s.

Currently there's no way to validate if all `UnsafeAccessor`s in an assembly are valid without running all of them and seeing if they throw.

Being able to check if they can be resolved with `ilverify` would help in reducing accidental breaks when changing stuff used by `UnsafeAccessor`s.

cc @jkotas @AaronRobinsonMSFT Would such option make sense? I wanted to replace a circular dependency between 2 DLLs in a project I work on but I realised that ILVerify (which we use to verify the references) would no longer report errors then.

Start by locating ILVerify's command-line option handling and its existing UnsafeAccessor or assembly verification tests. Read how current reference errors are reported, then define the flag's expected behavior: it should validate UnsafeAccessor resolution without executing the accessors and report invalid references. Confirm the behavior with focused ILVerify tests.

Written by the indexing model from the issue text.