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.