Suppress valgrind warning about fUniqueID unitialized memory used.

This is done by taking the check of fUniqueID and the assignment to fBuits
into a separate function and by including this function in the header
*but* marking it as "noinline" to prevent its inlining and preserve a
way to suppress the valgrind warning.

Introducing the macro R__NEVER_INLINE which expands to

   inline __attribute__((noinline))

The performance degradation is less than 10% for a case doing
only creation and destruction of TObjects.