Add prototype ccc rewrite.

 - Entry point is tools/ccc/xcc until we are a functional replacement
   for ccc.

This is highly experimental (FIXME/LOC ratio of 3.4%), quite crufty,
and barely usable (and then only on my specific Darwin). However, many
of the right ideas are present, and it already fixes a number of
things gcc gets wrong.

The major missing component is argument translation for tools
(translating driver arguments into cc1/ld/as/etc. arguments). This is
a large part of the driver functionality and will probably double the
LOC, but my hope is that the current architecture is relatively
stable.

Documentation & motivation to follow soon...

llvm-svn: 61739