MarkEOLs should only be true for clang-cl.exe.

Summary:
https://llvm.org/bugs/show_bug.cgi?id=27396

This fixes an issue in response files where "\r\n" was being interpreted
as two EOL markers (i.e. we consumed the '\r' as terminating the
previous token, and then parsed the '\n' as a significant EOL). This
breaks response files where joined arguments get split across multiple
lines (like "-x\r\nc"). I also fixed an accidental issue in the
response-file.c test, where the response file is appended to, instead of
being overwritten.

Reviewers: rnk

Subscribers: danalbert, llvm-commits

Differential Revision: http://reviews.llvm.org/D19289

llvm-svn: 266840