[clang][CodeGen] Break up TargetInfo.cpp [3/8]

Don't derive from `DefaultTargetCodeGenInfo`.
This class is going to stay in `TargetInfo.cpp`, whereas its derivants
are going to be moved to separate translation units. Just derive from
the base `TargetCodeGenInfo` class instead.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D148091