Discovered while working on a patch to move alignment in
@llvm.memcpy/move/set from an arg into parameter attributes.
The current implementations of AttributeSet::removeAttribute() and
AttributeList::removeAttribute crash when attempting to remove the
alignment attribute. Currently, these implementations add the
to-be-removed attributes to an AttrBuilder and then remove
the builder from the list/set. Alignment is special in that it
must be added to a builder with an integer value for the alignment;
attempts to add alignment to a builder without a value is an error.
This change fixes the removeAttribute implementations for AttributeSet and
AttributeList to make them able to remove the alignment, and other similar,