Add a '-R' option, which is similar to '-r', except that the relocated directory, if exists, will be removed entirely
before running the test suite.  A usage example looks like this:

test $ ./dotest.py -A x86_64 -R /tmp/x86_64 &
test $ ./dotest.py -A i386 -R /tmp/i386 &

where we would want to run the x86_64 and i386 archs concurrently but relocate the test suite to different directory
hierarchies in order not to stump on each other's intermediate files.

llvm-svn: 155491