driver: Add a `--rsp-quoting` flag to pick response file quoting.

Currently, clang-cl always uses Windows style for unquoting, and clang always
uses POSIX style for unquoting.

With this flag, it's possible to change these defaults.

In general, response file quoting should match the shell the response file is
used in.  On Windows, it's possible to run clang-cl in a bash shell, or clang in
cmd.exe, so a flag for overriding the default behavior is natural there.

On non-Windows, Windows quoting probably never makes sense (except maybe in
Wine), but having clang-cl behave differently based on the host OS seems
strange too.  So require that people who want to use posix-style response
files with clang-cl on non-Windows pass --rsp-quoting=posix.

http://reviews.llvm.org/D19425

llvm-svn: 267474

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