adjust the prototypes of a bunch of decl processing methods to take

adjust the prototypes of a bunch of decl processing methods to take
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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@52871 91177308-0d34-0410-b5e6-96231b3b80d8