Move TF Feedback linked varyings into D3D-only.

The LinkedVarying structure is a D3D-specific type, and the GL
back-end doesn't need the extra info. Isolate this into the D3D
back-end so we can clean up the Impl inteface.

BUG=angleproject:1123

Change-Id: I76d77ac505876d865e3e02f47acbfd6665a9507e
Reviewed-on: https://chromium-review.googlesource.com/293764
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/renderer/gl/ProgramGL.cpp b/src/libANGLE/renderer/gl/ProgramGL.cpp
index fbe8c50..a103261 100644
--- a/src/libANGLE/renderer/gl/ProgramGL.cpp
+++ b/src/libANGLE/renderer/gl/ProgramGL.cpp
@@ -64,9 +64,11 @@
     return gl::Error(GL_INVALID_OPERATION);
 }
 
-LinkResult ProgramGL::link(const gl::Data &data, gl::InfoLog &infoLog,
-                           gl::Shader *fragmentShader, gl::Shader *vertexShader,
-                           int *registers, std::vector<gl::LinkedVarying> *linkedVaryings,
+LinkResult ProgramGL::link(const gl::Data &data,
+                           gl::InfoLog &infoLog,
+                           gl::Shader *fragmentShader,
+                           gl::Shader *vertexShader,
+                           int *registers,
                            std::map<int, gl::VariableLocation> *outputVariables)
 {
     // Reset the program state, delete the current program if one exists