Move the state bits in DeclRefExpr out of the pointer union and into

a bitfield in the base class. DREs weren't using any bits here past the
normal Expr bits, so we have plenty of room. This makes the common case
of getting a Decl out of a DRE no longer need to do any masking etc.

Also, while here, clean up code to use the accessor methods rather than
directly poking these bits, and provide a nice comment for DREs that
includes the information previously attached to the bits going into the
pointer union.

No functionality changed here, but DREs should be a tad faster now.

llvm-svn: 130666