Driver: Change default use of "clang" compiler.

 - Don't default to using clang for C++ (use -ccc-clang-cxx to
   override).

 - Default to only using clang on i386 and x86_64 (use
   -ccc-clang-archs "" to override).

 - <rdar://problem/6712350> [driver] clang should not be used on
   powerpc by default
 - <rdar://problem/6705767> driver should default to -ccc-no-clang-cxx

I plan to add a warning that we are not using the clang compiler for
the given compilation so that users do not think clang is being used
in situations it isn't.

This change is motivated by the desire to be able to drop clang into a
build and have things "just work", even if it happens to get used to
compile C++ code or code for an architecture we don't support yet.

llvm-svn: 67640