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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298513 91177308-0d34-0410-b5e6-96231b3b80d8