Ensure the shared symbol table levels are read-only to make multi-threading safe.  Also removed inadvertent extra copies of the symbol table shared across all stages.



git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@22939 e7fa87d3-cd2b-0410-9028-fcbf551c1848