There are two valid values for this: 'swift' and 'swiftc'. This flag must be
specified as the first option; otherwise, it will be ignored. This flag allows
the caller of the driver to force 'swift' to behave as 'swiftc', or vice versa,
and is useful in situations where the name of the executable cannot be changed.
Swift SVN r20656
This flag is now obsoleted by the interactive driver and simply
complicates understanding the command-line parsing. Making it an error
to force users to move also allows us to reuse the flag in the future if
we like.
Swift SVN r20641
We were already effectively doing this everywhere /except/ when building
the standard library (which used -O2), so just use the model we want going
forward.
Swift SVN r20455
... and use them to start diagnosing unsupported mode flag and
DriverKind combinations (e.g. swifti -c, swiftc -i). Also hide the
unsupported options from -help.
Swift SVN r20452
This allows swiftFrontend to drop its dependency on swiftDriver, and could
someday allow us to move the integrated frontend's option parsing out of
swiftFrontend (which would allow other tools which use swiftFrontend to
exclude the option table entirely).
Swift SVN r19824
