[PATCH] Skip Ubsan's vptr checks on constructors and destructors.

[Richard Smith]

It's not OK to just turn off this checking for everyone. It's also not appropriate to blindly turn off all checks in a constructor or destructor. This also doesn't do the right thing for code invoked from a constructor or destructor.

Instead, I'd suggest you do the following:

 * When a complete object constructor starts for a polymorphic class type, write a record to a (thread-local) stack indicating the address, size, and type_info of the complete object.
 * When the sanitizer runtime detects a problem, check this side-table. If the access is to a subobject that would be present once the complete object is constructed, suppress the diagnostic (possibly based on a runtime flag).

http://reviews.llvm.org/D4528