Allow assume bounds casts for function pointers

[kkjeer]

Fixes #855

This PR allows _Assume_bounds_casts to be used to convert an unchecked function pointer (including NULL) to a checked function pointer. The unchecked function pointer pointee type must be compatible with the checked function pointer pointee type.

For example, this will be allowed:

void test(int(*f)(int)) {
    ptr<int(int)> safe = _Assume_bounds_cast<ptr<int(int)>>(f);
}

This will not be allowed:

void test(int(*f)(int)) {
    ptr<void(double)> bad = _Assume_bounds_cast<ptr<void(double)>>(f);
}

Testing:

• Added tests in checkedc/407
• Passed manual testing on Windows
• Passed automated testing on Windows/Linux

[dtarditi]

Looks good!