Add support for function pointers in ilverify (#57163)

Add support for function pointers in ilverify (#57163)

* Provide logic verifying access to function pointer.

Extend AccessVerificationHelpers.CanAccess method so that it is
able to handle FunctionPointerType by making sure that a current class
can access all the types returned or accepted by the method pointed
by the given function pointer.

Fix #43502

* Add support for function pointers in ilverify importer

* Add ilverify tests for function pointers

* Trim trailing whitespace

* Use public surface of FunctionPointerType

* Check both return type and arguments of signature

* Delete CanAccessMethodSignature and simplify

* Add a test case for inaccessible return type