Pass CGCXXABIs around directly.

In functions that only need to use the CGCXXABI member of a CodeGenTypes
class, pass that reference around directly rather than a reference to
a CodeGenTypes class.

This makes the actual dependence on CGCXXABI clear at the call sites.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@192052 91177308-0d34-0410-b5e6-96231b3b80d8