Tool selection is primarily done by checking the executable (= symlink) name.
But sometimes (e.g. if the tool symlink is not there) it's useful to have an option for selecting the tool.
The selection option (e.g. -sil-opt) must be the first argument of swift-frontend.
The layouts of resilient value types shipped in the Swift 5 standard library
x and overlays will forever be frozen in time for backward deployment to old
Objective-C runtimes. This PR ensures that even if the layouts of these types
evolve in the future, binaries built to run on the old runtime will continue
to lay out class instances in a manner compatible with Swift 5.
Fixes <rdar://problem/45646886>.
I am going to use this in bug reducer for debugging runtime crashes. I just
found the branch and cleaned it up, so I fugred I would commit it sooner rather
than after I lost the branch again.
