Undo the unique_ptr'fication of CodeGenABITypes::CGM introduced in r248762.

include/clang/CodeGenABITypes.h is in meant to be included by external
users, but using a unique_ptr on the private CodeGenModule introduces a
dependency on the type definition that prevents such a use.

NFC

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249328 91177308-0d34-0410-b5e6-96231b3b80d8