Remove driver support for Hexagon V2 and V3

Driver will now error when trying to compile for V2 or V3.

Removal of V2 and V3 support will allow us to simplify the hexagon
back-end.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@176859 91177308-0d34-0410-b5e6-96231b3b80d8