tsan: remove implicit memcpy in MutexSet::Desc::operator=()

The default compiler-generated MutexSet::Desc::operator=()
now contains memcpy() call since Desc become bigger.
This fails in debug mode since we call interceptor from within the runtime.
Define own operator=() using internal_memcpy().
This also makes copy ctor necessary, otherwise:
tsan_mutexset.h:33:11: warning: definition of implicit copy constructor for
'Desc' is deprecated because it has a user-declared copy assignment operator
And if we add copy ctor, we also need the default ctor
since it's called by MutexSet ctor.

Depends on D107911.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D107959