[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