[flang][driver] Support parsing response files

Add support for reading response files in the flang driver. Response
files contain command line arguments and are used whenever a command
becomes longer than the shell/environment limit. Response files are
recognized via the special "@path/to/response/file.rsp" syntax, which
distinguishes them from other file inputs.

This patch hardcodes GNU tokenization, since we don't have a CL mode for
the driver. In the future we might want to add a --rsp-quoting command
line option, like clang has, to accommodate Windows platforms.

Differential Revision: https://reviews.llvm.org/D124846