Use os.path.realpath when tracking the cwd.

This is needed by TestCases/Posix/coverage-direct.cc

The problem is that the test does:

mkdir <dir>
cd <dir>
cd ..
rm -rf <dir>
<more commands>

the current directory currently looks like "/.../<dir>/../" which
doesn't exist when dir is deleted.

at some point we should probably switch to using the os current
directory (specially if we want to add subshell), but this is a small
incremental improvement.

llvm-svn: 299113