Make home_directory look in the password database in addition to $HOME.

This is something of an edge case, but when the $HOME environment
variable is not set, we can still look in the password database
to get the current user's home directory.

Added a test for this by getting the value of $HOME, then unsetting
it, then calling home_directory() and verifying that it succeeds
and that the value is the same as what we originally read from
the environment.

llvm-svn: 298513