This change moves the code generation at link time from source code to
SPIR-V.  As a result, transform feedback extension and emulation paths
are more similarly handled before SPIR-V transformation (they both store
information identically in the ShaderInterfaceVariableInfoMap).

This change gets rid of the @@ XFB-OUT @@ marker.  With no source code
generation at link time, shader compilation can be moved to
glCompileShader time.

Bug: angleproject:4888
Change-Id: I8cdb89c22b57ce48cf5d226b8e41622d9d550d46
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2713269
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>