This makes two changes for the linking step:

* allows symbol names in TOutputGLSLBase::writeVariableType
* stores a list of inactive varying names in VaryingPacking

Bug: angleproject:2456
Change-Id: Id651721a008c049508303d18c45c6b96c76114d9
Reviewed-on: https://chromium-review.googlesource.com/1014707Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>