Create linkage symbol nodes in the AST so a linker has access to all global objects that were declared, for error checking, etc.  Use it now for all ins/outs/uniforms as well as gl_VertexID and gl_InstanceID.

Also fixed a confusing name and added more 'const'.


git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@22142 e7fa87d3-cd2b-0410-9028-fcbf551c1848