Remove GlslangToSpv.h dependency on intermediate.h

GlslangToSpv.h only declares functions with opaque references to to
TIntermediate, for which a simple "class TIntermediate;" declaration is
adequate. This means that Include/intermediate.h no longer needs to be
installed as part of glslang's public interface.

SPIRV/GlslangToSpv.h

@@ -35,19 +35,15 @@
 
 #pragma once
 
-#if defined(_MSC_VER) && _MSC_VER >= 1900
-    #pragma warning(disable : 4464) // relative include path contains '..'
-#endif
-
 #include "SpvTools.h"
-#include "glslang/Include/intermediate.h"
 
 #include <string>
 #include <vector>
 
 #include "Logger.h"
 
 namespace glslang {
+class TIntermediate;
 
 void GetSpirvVersion(std::string&);
 int GetSpirvGeneratorVersion();