implement a new clang-cc option -dump-build-information=filename which causes the

compiler to dump random stuff from the build into the file.  Right now this
amounts to dumping command line arguments and diagnostics to the file.

The idea is that you can set an envvar, do a world build of an OS, then grep 
through all the logs for interesting things or something.

Daniel, please wire the driver up to do something with this.

llvm-svn: 69386