Devirtualize Attr and all subclasses.

We know all subclasses in tblgen so just generate a giant switch for
the few virtual methods or turn them into a member variable using spare
bits. The giant jump tables aren't pretty but still much smaller than
a vtable for every attribute, shrinking Release+Asserts clang by ~400k.

Also halves the size of the Attr base class. No functional change
intended.

llvm-svn: 232726