`ELFFile<ELFT>` has many methods that take pointers,
though they assume that arguments are never null and
hence could take references instead.
This patch performs such clean-up.
Differential revision: https://reviews.llvm.org/D87385
This change moves elfabi related code to llvm/InterfaceStub library
so it can be shared by multiple llvm tools without causing cyclic
dependencies.
Differential Revision: https://reviews.llvm.org/D85678