Implement first TD-based usage of attributes.

Currently, there are two effective changes:

 - Attr::Kind has been changed to attr::Kind, in a separate namespace
   rather than the Attr class. This is because the enumerator needs to
   be visible to parse.
 - The class definitions for the C++0x attributes other than aligned are
   generated by TableGen.

The specific classes generated by TableGen are controlled by an array in
TableGen (see the accompanying commit to the LLVM repository). I will be
expanding the amount of code generated as I develop the new attributes system
while initially keeping it confined to these attributes.

llvm-svn: 106172