[FuzzMutate] Move LLVM module (de)serialization from FuzzerCLI -> IRMutator. NFC

These are not directly related to the CLI, and are mostly (always?) used when
mutating the modules as part of fuzzing.

Motivation: split FuzzerCLI into its own library that does not depend on IR.
Subprojects that don't use IR should be be fuzzed without the dependency.

Differential Revision: https://reviews.llvm.org/D125080