the single attribute they look at by reference instead of by pointer.
This is a subtle indicator that they take the specified attribute, not
a whole list of them.

This also make HandleExtVectorTypeAttribute work the same way as the rest
of the attributes, adds some comments etc. No functionality change.

llvm-svn: 52871

SemaDeclAttr, and do some cleanups.

llvm-svn: 52844

llvm-svn: 52792