Renamed class GRConstants => GRExprEngine.

This was done with a Perl script, and will result in 80 col. violations that
I will gradually fix up.

llvm-svn: 47070

have been visited in a path.  Added GRBlockCounter as an item to be
enqueued to the worklist.

Modified "ProcessBranch" in GRConstants to prune branches with symbolic
conditions that have been already taken.

llvm-svn: 47010