It turns out the problem was with the full length of the argument list
in characters (bytes), not just the /number/ of arguments. Defeat this
by using paths relative to the temporary directory we're using.
rdar://problem/25617239, hopefully for the last time.
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.
