mesa: don't advertise bogus GL_ARB_shading_language_120 extension

mesa: don't advertise bogus GL_ARB_shading_language_120 extension

Instead of using the invalid GL_ARB_shading_language_120 extension to
determine the GLSL version, use a new ctx->Const.GLSLVersion field.
Updated the intel and r600 drivers, but untested.

See fd.o bug 29910

NOTE: This is a candidate for the 7.9 branch (but let's wait and see if
there's any regressions).