When generating a compiler invocation in driver::createCompilerInvocation()
we end up using filelists if the number of inputs is > 128 (to work around
command line arg limits). We never actually write them out though, and so
fail when parsing the frontend arguments that reference them.
As this function is called frequently by SourceKit and command line limits
aren't a concern here, this patch makes the 128 threshold value configurable
via a new -driver-filelist-threshold option. This is set to its maximum value
in driver::createCompilerInvocation() to ensure filelists aren't used. This
new option makes the existing -driver-use-filelists (that forces filelists to
be used) redundant as it's now equivalent to -driver-filelist-threshold=0.
Resolves rdar://problem/38231888
