Glslang wrapper was trying to identify whether the varying is declared
in each of the out/in shaders by looping through the macro symbols that
need replacement.

This change instead adds stage information to PackedVarying assigned
when collecting varyings.  Glslang wrapper then simply tests the
bitfield for the stages of interest.

Bug: angleproject:3571
Change-Id: I29614e3e62d7df88e413c1732ac04e24243f167a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1954677Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>