[Sema] Make extended vectors of `bool` an error.

In OpenCL, `bool` vectors are a reserved type, and are therefore
illegal.

Outside of OpenCL, if we try to make an extended vector of N `bool`s,
Clang will lower it to an `[N x i1]`. LLVM has no ABI for bitvectors, so
lots of operations on such vectors are thoroughly broken. As a result,
this patch makes them illegal in everything else, as well. :)

Differential Revision: http://reviews.llvm.org/D15721

llvm-svn: 259011