Cleanup/comment IdentifierInfo::get.

 - Shaves off a few instructions on x86_64.

One notable change: this intentionally stops setting the II->Entry
field of IdentifierInfo's retrieved by the ExternalLookup method. This
is to maintain the invariant that .getName() has a constant value for
any given IdentifierInfo. IRgen currently relies on this; which is
quite questionable but will be cleaned up in time.

Apologies for the lack of a test case; there really isn't a good way
to make one. As IRgen will eventually be fixed to not rely on this, we
can survive without one.

llvm-svn: 66316