swift234

[driver] Added a hidden -force-single-frontend-invocation option.

This sets OutputInfo::CompilerMode to a new SingleCompile mode, which in turn
causes the driver to create a single CompileJobAction for all inputs. This mode
also prevents the creation of a MergeModuleJobAction, since we can rely on the
frontend to generate a full swiftmodule.

This mode will allow the driver to properly compile the stdlib; if this mode is
necessary for other inputs, we can rename and publicize this option.

Swift SVN r12612