This patch intends to clean up some issues in VaryingPacking
to implement geometry shader easiler.

1. Use emplace_back() instead of push_back() when necessary.
2. Remove unnecessary parameter in VaryingPacking::packUserVaryings().
3. Remove the assignment of semanticIndex and only handle them in D3D11
   back-ends.

BUG=angleproject:1941

Change-Id: Ia09c07f01dc442ce95cb4984e4b768d0c79872c7
Reviewed-on: https://chromium-review.googlesource.com/1128576Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>