Fix checking of function pointer casts involving lvalues.

[dtarditi]

A programmer reported that an indirect function call via a const member resulted in an unexpected compiler error message (Github issue #481). The problem was that the compiler was too restrictive when handling lvalue-to-rvalue casts. The lvalue-to-rvalue casts removes qualifiers on the type. The compiler was checking for the exact type.

I reworked the code for checking a function pointer cast expression. The existing code was hard to follow and there were some bugs in it. The code now has 3 distinct steps:

1. Handle casts to checked function pointers that are valid by default.
   This includes lvalue-to-rvalue casts and bounds-safe interface casts.
2. Otherwise, skip over value-preserving casts and value-preserving
   operations involving function pointers. Stop when you reach an expression E
   that has checked pointer type or that isn't a value-preserving cast
   or cast-like operation.
3. Check that E is guaranteed to produce a valid function pointer:

• E is a reference to a function name.
• E is a null pointer.
• E is checked pointer.

Testing:

• Add additional tests to the Checked C repo for function pointer casts.
• Passed local testing on Windows.
• Passed automated testing on Linux.