Fix an assertion failure in unwrapSugar

An assertion in Qualifiers::addObjCLifetime fails when the ObjC lifetime
bits are already set.

Instead of calling operator+=, call addConsistentQualifiers, which
allows the lifetime bits to be set again as long the new value doesn't
conflict with the old value.

This fixes https://github.com/llvm/llvm-project/issues/61419.

Differential Revision: https://reviews.llvm.org/D147263