swift234

[driver] Added initial infrastructure for generating a Command which merges swiftmodule files.

Added a MergeModuleJobAction, which will be translated into a Command which merges swiftmodule files.
Added a MergeModule subclass of Tool, which will generate a Command which merges swiftmodule files. (Currently, it generates a Command which invokes “swift_driver -frontend -help”.)
Added ToolChain-level support for getting the MergeModule Tool.

Nothing produces MergeModuleJobActions yet; adding that to the Action dependency graph should be enough to get this going.

Swift SVN r12491