Support: Add llvm::sys::fs::copy_file

A function to copy one file's contents to another.
Frontend: Add a CC1 flag to dump module dependencies to a directory

This adds the -module-dependency-dir to clang -cc1, which specifies a
directory to copy all of a module's dependencies into in a form
suitable to be used as a VFS using -ivfsoverlay with the generated
vfs.yaml.

This is useful for crashdumps that involve modules, so that the module
dependencies will be intact when a crash report script is used to
reproduce a problem on another machine.

We currently encode the absolute path to the dump directory, due to
limitations in the VFS system. Until we can handle relative paths in
the VFS, users of the VFS map may need to run a simple search and
replace in the file.