swift234

Revert the API change of r5893

This commit merges EnvironmentVariables and CommandLineArguments into a single
swift.Process global variable.  POSIX-dependent parts are available as
extension with "import POSIX".

Original message:

stdlib: split the Process class into a POSIX-dependent (EnvironmentVariables)
and POSIX-independent (CommandLineArguments) parts


Swift SVN r6331