swift234

Add support for whole-module optimizations for SIL files.

To invoke the front-end on a SIL with whole-module optimizations enabled, execute:
swiftc -frontend myfile.sil

To invoke the front-end on a SIL without whole-module optimizations enabled, add a -primary-file option:
swiftc -frontend -primary-file myfile.sil

To invoke a sil-opt with whole-module optimizations enabled, use the -wmo option:
sil-opt myfile.sil -wmo

This change was need to be able to write SIL unit tests which should be compiled in the WMO mode.

Swift SVN r31862