This bug will become moot if/when we stop exposing flags to the
customer that allow them to choose specific (sets of) collectors.

As it currently stands, the following is true of the following set of
flags that decide the choice of collector(s):
   -Xincgc, -Xconcgc, -XX:+UseParallelGC, -XX:+UseParNewGC


. -XX:+UseParallelGC overrides all other flags in the above set
. -Xingc overrides all flags other than -XX:+UseParallelGC
. -Xconcgc and -XX:+UseParNewGC don't override each other and
  can be used in all combinations except when overridden by the
  flags in earlier bullets

In addition there may be potentially interactions with the
ParallelGCThreads flag (and potentially other such "shared flags")
which may need to be looked at.

This bug is being filed as a place holder to try and reduce potential
customer confusion wrt these GC flags.

Note that in general this is a problem not just with GC flags
but all the myriad XX flags. At some point it might be a good idea to
think through these interactions and try and work towards reducing
or at least making clear rules on these interactions. That however
is, in general, easier said than done.

However, for the handful of GC flags mentioned above it may be
relatively straightforward to devise and enforce a uniform rule
(for instance one based on the order of potentially conflicting
flags on the command-line) regarding interactions between them.
It may be a good ides to do at least that.