Fix attribute layout location linking.

Fix attribute layout location linking.

When a vertex attribute has a GLSL layout location qualifier, it takes
precedence over the binding location provided through the
glBindAttribLocation API call.

OpenGL ES 3.0.5 spec:
"If an active attribute has a binding explicitly set within the shader
text and a different binding assigned by BindAttribLocation, the
assignment in the shader text is used."

Change-Id: If0bc0dc01a8ff6189703f2be26f1938fbff5f5ae
Reviewed-on: https://swiftshader-review.googlesource.com/20168
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>