Use alias tag to check if ext and core funcs are same

Use alias tag to check if ext and core funcs are same

OpenGL API registry commands have <alias> tag that can be used to
determine if extension function can be treated as an alias to a core
entry point.

This change updates scripts to take advantage of alias tag to correctly
determine if ext and core entry points can be folded into one. This
fixes issues with EXT_direct_state_access where behavior has changed
between EXT and core versions.

VK-GL-CTS issue: 368
Components: Framework
Change-Id: I6addfdca02c778b7a44db1cb81d5a88aae6cf273