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

This commit breaks up CodeGen/TargetInfo.cpp into a set of *.cpp files,
one file per target. There are no functional changes, mostly just code moving.

Non-code-moving changes are:
* A virtual destructor has been added to DefaultABIInfo to pin the vtable to a cpp file.
* A few methods of ABIInfo and DefaultABIInfo were split into declaration + definition
  in order to reduce the number of transitive includes.
* Several functions that used to be static have been placed in clang::CodeGen
  namespace so that they can be accessed from other cpp files.

RFC: https://discourse.llvm.org/t/rfc-splitting-clangs-targetinfo-cpp/69883

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D148094