[CodeGen] Add initial support for union members in TBAA

The basic idea behind this patch is that since in strict aliasing
mode all accesses to union members require their outermost
enclosing union objects to be specified explicitly, then for a
couple given accesses to union members of the form

p->a.b.c...
q->x.y.z...

it is known they can only alias if both p and q point to the same
union type and offset ranges of members a.b.c... and x.y.z...
overlap. Note that the actual types of the members do not matter.

Specifically, in this patch we do the following:

* Make unions to be valid TBAA base access types. This enables
  generation of TBAA type descriptors for unions.

* Encode union types as structures with a single member of a
  special "union member" type. Currently we do not encode
  information about sizes of types, but conceptually such union
  members are considered to be of the size of the whole union.

* Encode accesses to direct and indirect union members, including
  member arrays, as accesses to these special members. All
  accesses to members of a union thus get the same offset, which
  is the offset of the union they are part of. This means the
  existing LLVM TBAA machinery is able to handle such accesses
  with no changes.

While this is already an improvement comparing to the current
situation, that is, representing all union accesses as may-alias
ones, there are further changes planned to complete the support
for unions. One of them is storing information about access sizes
so we can distinct accesses to non-overlapping union members,
including accesses to different elements of member arrays.
Another change is encoding type sizes in order to make it
possible to compute offsets within constant-indexed array
elements. These enhancements will be addressed with separate
patches.

Differential Revision: https://reviews.llvm.org/D39455

llvm-svn: 319413