SPV: Plumb through XFB buffer and stride information.

Also, only emit this XFB information where the SPIR-V spec says
it should be emitted: essentially, on objects.

This and the previous commit together fix #1185.

glslang/MachineIndependent/localintermediate.h

@@ -583,6 +583,7 @@ public:
         xfbBuffers[buffer].stride = stride;
         return true;
     }
+    unsigned getXfbStride(int buffer) const { return xfbBuffers[buffer].stride; }
     int addXfbBufferOffset(const TType&);
     unsigned int computeTypeXfbSize(const TType&, bool& containsDouble) const;
     static int getBaseAlignmentScalar(const TType&, int& size);