In the emulation path, it's ensured that the generated code references
the I/O block field correctly (using the instance name if provided, and
without it otherwise).

In the extension path, the info map is augmented with an array of xfb
decorations for its fields.  Then when `OpDecorate %IOBlockId Block` is
encountered, the transform feedback decorations on the fields are
inserted:

    OpMemberDecorate %IOBlockId MemberN XfbBuffer buffer
    OpMemberDecorate %IOBlockId MemberN XfbStride stride
    OpMemberDecorate %IOBlockId MemberN Offset    offset

Future work includes removing the duplicate varying added for
gl_PointSize and use this mechanism to decorate gl_PerVertex directly.

Bug: angleproject:3606
Change-Id: I6fed0b1ee7245fe695337043b40b281fb01a1fb0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2599953Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>