Adjust Attr representation so that changes to documentation don't affect

how we parse source code.

Instead of implicitly opting all undocumented attributes out of '#pragma
clang attribute' support, explicitly opt them all out and remove the
documentation check from TableGen.

(No new attributes should be added without documentation, so this has
little chance of backsliding. We already support the pragma on one
undocumented attribute, so we don't even want to enforce our old
"rule".)

No functionality change intended.

llvm-svn: 341009