[Attributor] Make unimplemented method pure virtual.
[Attributor] Make unimplemented method pure virtual.

Otherwise the compiler mistakes it for a vtable anchor.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@371298 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm