Revert multiple patches based on "Introduce CfgTraits abstraction"

These logically belong together since it's a base commit plus
followup fixes to less common build configurations.

The patches are:

Revert "CfgInterface: rename interface() to getInterface()"

This reverts commit a74fc481.

Revert "Wrap CfgTraitsFor in namespace llvm to please GCC 5"

This reverts commit f2a06875.

Revert "Try to make GCC5 happy about the CfgTraits thing"

This reverts commit 03a5f7ce.

Revert "Introduce CfgTraits abstraction"

This reverts commit c0cdd22c.