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.
