Improvement to the AST traversal infrastructure.

Improvement to the AST traversal infrastructure.

Note:  This affects any downstream consumers of glslang's traverser.  Let me know if there are any issues.  Essentially, you will need to change the functions that were pointed to into overriding base-class members instead.  See the examples in this check in.

git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@24740 e7fa87d3-cd2b-0410-9028-fcbf551c1848