Add new tool 'diag-build' for showing enabled warnings in a project.

diag-build acts as a wrapper for 'diagtool show-enabled', in the same way
that scan-build acts as a wrapper for the static analyzer. The common case is
simple: use 'diag-build make' or 'diag-build xcodebuild' to list the warnings
enabled for the first compilation command we see. Other build systems require
you to manually specify "dry-run" and "use $CC and $CXX"; if there is a build
system you are interested in, please add it to the switch statement.

diag-build is fairly stupid right now, but it serves its basic purpose.
Hopefully it can grow to meet any additional requirements.

llvm-svn: 158682