Finishes up https://bugs.swift.org/browse/SR-280. At this point, nothing
should be passing O(N) arguments on the command line, where N is the
number of input source files, unless N is small.
(There are other inputs which are passed through to subtools, and these
are not put into filelists. That's fine.)
With this, we're out of the business of passing large numbers of input
files on the command line to the frontend, which means we no longer
overflow argv with a mere 1100 input files under whole-module optimization.
In order to make sure this doesn't happen again, I'd like to also get
this working for
- swiftmodule inputs to the merge-module build phase
- /output/ files for multithreading single-frontend builds (WMO)
- object file inputs to the linker on OS X (response files for binutils
ld have different quoting rules)
Part 3 of https://bugs.swift.org/browse/SR-280.
