Improve ccc:

 - Support environment variable CCC_LANGUAGES to control which
   languages clang is invoked on. If unset clang is invoked for all
   languages, otherwise CCC_LANGUAGES should be a comma separated list
   of the languages (as accepted by -x) for which clang should be
   invoked. Useful for only building C and Objective-C parts of a
   project with clang, for example.

 - Add environment variable CCC_FALLBACK. If set and non-empty then
   ccc will try and compile using the regular compiler if compilation
   with clang fails.

 - A few other tweaks to add options, flush stdout, recognize .mm as
   objective-c++, and infer languages for compile+link style
   invocations.

llvm-svn: 55547