[clang][CodeGen] Break up TargetInfo.cpp [4/8]

Remove `getABIInfo` overrides returning references to target-specific
implementations of `ABIInfo`.
The methods may be convenient, but they are only used in one place and
prevent from `ABIInfo` implementations from being put into anonymous
namespaces in different cpp files.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D148092