[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
ObjectiveC migration. migration to NS_ENUM/SN_OPTION
- wip.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186604 91177308-0d34-0410-b5e6-96231b3b80d8
Turn Driver::CCCIsCXX and CCCIsCPP into a single Mode enum,
and add a new option --driver-mode= to control it explicitly.

The CCCIsCXX and CCCIsCPP flags were non-overlapping, i.e. there
are currently really three modes that Clang can run in: gcc, g++
or cpp, so it makes sense to represent them as an enum.

Having a command line flag to control it helps testing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186605 91177308-0d34-0410-b5e6-96231b3b80d8