Preprocessor:  Implement skipping of macros that evaluate to nothing while evaluating expressions.  They had been causing early termination of an expression's evaluation.

Also includes some unrelated updates of documentation.


git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@24420 e7fa87d3-cd2b-0410-9028-fcbf551c1848