Swift: Update default build flags

Fully-optimized builds should be using whole-module optimizations(WMO)
to get all the optimizations the compiler can do for a given module.
As such, it makes sense for the release builds to pass
`-whole-module-optimization` or `-wmo` to the compiler by default.
`-whole-module-optimization` and `-wmo` are aliased and have the same
impact on the build.

Removing `-incrementa' from the `CMAKE_Swift_CREATE_*` variable:
WMO is incompatible with incremental builds, so it is removed to avoid
warnings from the Swift compiler.

Pass `-num-threads` to the driver in `CMAKE_Swift_CREATE_*`:
WMO doesn't use the `-j` flag, but instead uses `-num-threads` to get
parallelism. The two flags are applied in mutually exclusive contexts,
so `-j N` is a no-op in WMO, while `-num-threads` is a no-op in other
modes. Passing both at the same time will catch both cases without
negatively impacting the other case.

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