Use gcc's rules for parsing gcc-style response files

Use gcc's rules for parsing gcc-style response files

In gcc, \ escapes every character in response files. It is true that this makes
it harder to mention Windows files in rsp files, but not doing this means clang
disagrees with gcc, and also disagrees with the shell (on non-Windows) which
rsp file quoting is supposed to match. clang isn't free to choose what to do
here.

In general, the idea for response files is to take bits of your command line
and write them to a file unchanged, and have things work the same way. Since
the command line would've been interpreted by the shell, things in the rsp file
need to be subject to the same shell quoting rules.

People who want to put Windows-style paths in their response files either need
to do any of:
* escape their backslashes
* or use clang-cl which uses cl.exe/cmd.exe quoting rules
* pass --rsp-quoting=windows to clang to tell it to use
  cl.exe/cmd.exe quoting rules for response files.

Fixes PR27464.
http://reviews.llvm.org/D19417

llvm-svn: 267556