This commit introduces new tags to specify how LLVMContainer should
deduplicate sets of equivalent functions, typically managed by an
`OpaqueFunctionPool`.
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.
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.