swift234

Add a -gnone option that can negate a previous -g option.

The name -gnone was chosen by analogy with -O and -Onone. Like -O/-Onone,
the last option on the command line wins.

The immediate use case for this is because we want to be able to run the
tests with -g injected into every command line, but some tests will fail
when debug info is included. Those particular tests can be explicitly marked
-gnone.

rdar://problem/18636307

Swift SVN r22777