Allow assume bounds casts for function pointers

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#865

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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