SPV: Give error on not assigning locations to I/O.

Also, provides an option to auto-assign locations.
Existing tests use this option, to avoid the error message,
however, it is not fully implemented yet.

glslang/MachineIndependent/linkValidate.cpp

@@ -860,7 +860,7 @@ int TIntermediate::checkLocationRange(int set, const TIoRange& range, const TTyp
     return -1; // no collision
 }
 
-// Accumulate locations used for inputs, outputs, and uniforms, and check for collisions
+// Accumulate bindings and offsets, and check for collisions
 // as the accumulation is done.
 //
 // Returns < 0 if no collision, >= 0 if collision and the value returned is a colliding value.