It can be enabled with the -num-threads <n> option.
Without this option there should be NFC.
When enabled, the LLVM IR is split into multiple modules: one module for each input file.
And for each module an output file is generated. All output files must be specified with -o options:
for each input file in the command line there must be an -o <outputfile> option.
LLVM compilation is performed on each module separately.
This means that the generated code is different than with regular -wmo.
But performance and code size should be approximately the same because important inter-file
optimizations are already done at SIL level (e.g. inlining, specialization).
There is still no support in the driver for this feature.
Swift SVN r25930
