Without this attribute, newer versions of llvm are not able to
remove calls to `ReadOnly` functions whose return value is unused.
This is instead the expected behavior for the functions that are
generated by `revng` with the `ReadOnly` attribute.
With llvm-12 thera are more stringent requirement on the propagation of
DebugLoc on CallInst and InvokeInst.
Various CallInst and InvokeInst created during lifting did not fulfill
these requirements, causing the Module to not verify() with llvm-12.
This commit fixes the problem, properly propagating the debug locations.
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.
Antonio Frighetto