Add a mixin class for piperuns that work on an LLVM module and need
both the model function and the `llvm::Function`. Pipe that inherit this
mixin will receive both the model and LLVM function in the
`runOnLLVMFunction` method.
This commit takes out of EnforceABI the part taking care of creating
wrappers for calls to helpers and promoting CSV to local variables.
This decoupling, enables to run -promote-csvs multiple times, for
instance after inlining.
Giacomo Vercesi