[Driver] Use LLVM's response file parser because it can read UTF-16

MSBuild writes response files as UTF-16 little endian with a byte order
mark.  With this change, clang will be able to read them, although we
still can't parse any of their flags.

Adds a UTF-16-LE response file with a BOM for testing.

Differential Revision: http://llvm-reviews.chandlerc.com/D1137

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186603 91177308-0d34-0410-b5e6-96231b3b80d8