Do not generate samplerBuffer for spirv1.6 and beyond

This type was removed from spirv1.6. If samplerBuffer is specified in
GLSL, generate textureBuffer. If samplerBuffer type is constructed,
just return the buffer.

Fixes #2956

Test/spv.1.6.separate.frag

@@ -0,0 +1,14 @@
+#version 400
+
+uniform sampler s;
+
+uniform textureBuffer             texBuffer;
+uniform itextureBuffer            itexBuffer;
+uniform utextureBuffer            utexBuffer;
+
+void main()
+{
+    samplerBuffer          (texBuffer, s);
+    isamplerBuffer         (itexBuffer, s);
+    usamplerBuffer         (utexBuffer, s);
+}