Fix false shader cache hits for ray tracing shaders

Fix false shader cache hits for ray tracing shaders

Same value ("unused for GL") returned by getShaderTypeName() for
ray tracing shader types might lead to invalid shader cache hits.
Second condition required to have a false hit: two shaders of
different type have same code.

Return unique names for shader types from glu::getShaderTypeName.

Affects:
 * dEQP*

Components: framework

VK-GL-CTS issue: 2224

Change-Id: Icec4448a9f1312884990e983e0828c2fe16f83bf