Implement 4 AMD-specific extensions.

- Support GL_AMD_shader_ballot (SPV_AMD_shader_ballot).
- Support GL_AMD_shader_trinary_minmax (SPV_AMD_shader_trinary_minmax).
- Support GL_AMD_shader_explicit_vertex_parameter
  (SPV_AMD_shader_explicit_vertex_parameter).
- Support GL_AMD_gcn_shader (SPV_AMD_gcn_shader).

glslang/Include/Types.h

@@ -405,6 +405,9 @@ public:
         smooth       = false;
         flat         = false;
         nopersp      = false;
+#ifdef AMD_EXTENSIONS
+        explicitInterp = false;
+#endif
         patch        = false;
         sample       = false;
         coherent     = false;
@@ -438,6 +441,9 @@ public:
     bool smooth       : 1;
     bool flat         : 1;
     bool nopersp      : 1;
+#ifdef AMD_EXTENSIONS
+    bool explicitInterp : 1;
+#endif
     bool patch        : 1;
     bool sample       : 1;
     bool coherent     : 1;
@@ -453,7 +459,11 @@ public:
     }
     bool isInterpolation() const
     {
+#ifdef AMD_EXTENSIONS
+        return flat || smooth || nopersp || explicitInterp;
+#else
         return flat || smooth || nopersp;
+#endif
     }
     bool isAuxiliary() const
     {
@@ -1518,6 +1528,10 @@ public:
             p += snprintf(p, end - p, "flat ");
         if (qualifier.nopersp)
             p += snprintf(p, end - p, "noperspective ");
+#ifdef AMD_EXTENSIONS
+        if (qualifier.explicitInterp)
+            p += snprintf(p, end - p, "__explicitInterpAMD ");
+#endif
         if (qualifier.patch)
             p += snprintf(p, end - p, "patch ");
         if (qualifier.sample)