This applies the same changes from Clang CFE r349065 to the Swift
frontend to unify how filenames, cmpilation directories and absolute
paths in filenames and path remappings are handled.
They are mandatory inlined anyway. Therefore the binary code of transparent functions is not used in most cases.
In case the address of a transparent function is taken, the main program (which deserialized the transparent function) generates code for it and treats it as shared function.
This reduces the stdlib code size by 2.8%.
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
