Replace uses of ImmutableSet in SymbolReaper with DenseSet. This was

motivated from Shark profiles that shows that 'markLive' was very
heavy when using --analyzer-store=region.  On my benchmark file, this
reduces the analysis time for --analyzer-store=region from 19.5s to
13.5s and for --analyzer-store=basic from 5.3s to 3.5s.  For the
benchmark file, this is a reduction of about 30% analysis time for
both analysis modes (a huge win).

llvm-svn: 80765