[Attributes] Fix crash when attempting to remove alignment from an attribute list/set

Summary:
 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,
attributes.

Reviewers: rnk, chandlerc, pete, javed.absar, reames

Reviewed By: rnk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41951

llvm-svn: 322735