[analyzer] Special-case bitfields when finding sub-region bindings.

Previously we were assuming that we'd never ask for the sub-region bindings
of a bitfield, since a bitfield cannot have subregions. However,
unification of code paths has made that assumption invalid. While we could
take advantage of this by just checking for the single possible binding,
it's probably better to do the right thing, so that if/when we someday
support unions we'll do the right thing there, too.

This fixes a handful of false positives in analyzing LLVM.

<rdar://problem/13325522>