add an equivalent to -fsanitize=local-init for C++ constructors

[thestinger]

C++ constructors can leave fields uninitialized. A sanitizer should be implemented to zero (note: not default initialize) any fields missing from the initializer list. The compiler can be relied upon to optimize out redundant initialization within the constructor itself in many cases.

[thestinger]

An alternative is extending the existing -fsanitize=local-init sanitizer to also zero before calling C++ constructors, since it cannot be assumed that they do full initialization.

[thestinger]

An alternative is extending the existing -fsanitize=local-init sanitizer to also zero before calling C++ constructors, since it cannot be assumed that they do full initialization.