[PATCH] Driver: add support for linker file lists

[Reid Kleckner]

This is a general problem: any subcommand we execute may overflow the system command line length limit.  I'd rather solve it transparently for -cc1 as well as linker invocations by exposing a command line length estimate from Support and then estimating our command line length in the driver before invoking the subcommand, optionally with a response file.  I know that Sony has an internal patch to do this for -cc1 invocations, so it's not just linker stuff.

http://reviews.llvm.org/D4104