[TableGen] Move generated *Attr class methods out of line

After this change, clang spends ~200ms parsing Attrs.inc instead of
~560ms. A large part of the cost was from the StringSwitch
instantiations, but this is a good way to avoid similar problems in the
future.

Reviewed By: aaron.ballman, rjmccall

Differential Revision: https://reviews.llvm.org/D76040

Author: rnk