Introduce and use the new `%target-abi` and `%target-import-type`
subsitutions in the IRGen tests. The former allows us to differentiate
between the Windows and SysV ABI differences and the latter for the
indirected import semantics required by PE/COFF. This makes all the
IRGen tests succeed on Windows.
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>.