Don't let test/Driver/no-canonical-prefixes.c form a symlink cycle the second time it runs.

The test makes %t.fake a symlink to %t.real by running `ln -sf %t.real
%t.fake`. If %t.fake already is a symlink to %t.real when this runs (e.g. if
the test has run before), then this effectively becomes `ln -sf %t.real %t.real`,
symlinking the directory to itself. At least on my mac, this leads to the
directory containing itself.

As fix, just remove %t.fake before creating the symlink. To clean up build dirs
on bots, also remove %t.real for a while.

https://reviews.llvm.org/D48224

llvm-svn: 334972