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
-wmo to the compiler by default.
Removing '-incremental ' from the
WMO is incompatible with incremental builds, so I am removing the
incremental flag from the 'CMAKE_Swift_CREATE_*' and adding it to the
default debug build flags.
-num-threads to the driver in
WMO mode doesn't use the
-j flag, but instead uses
get parallelism. The two flags are applicable in mutually exclusive
-j N is a no-op in WMO mode, and
-num-threads is a
no-op in other modes, so passing both at the same time will catch both