[X86] Refactor X86IndirectThunks.cpp to Accommodate Mitigations other than Retpoline

Introduce a ThunkInserter CRTP base class from which new thunk types can inherit, e.g., thunks to mitigate https://software.intel.com/security-software-guidance/software-guidance/load-value-injection.

Differential Revision: https://reviews.llvm.org/D76811