Add EnableIPRA to TargetOptions, and move the cl::opt -enable-ipra to TargetMachine.cpp

Avoid exposing a cl::opt in a public header and instead promote this
option in the API.
Alternatively, we could land the cl::opt in CommandFlags.h so that
it is available to every tool, but we would still have to find an
option for clang.

llvm-svn: 275348