Describe weak dependencies
    
    clang has a couple of indirect weak dependencies; installing clang
    without those will cause it to fail in certain use cases, but its
    description doesn't mention them which makes resolving this harder
    than it could be.
    
    This patch adds a brief description of the weak dependencies to the
    main clang package description.
    
    Closes: #1879410
    Signed-off-by: Stephen Kitt <skitt@redhat.com>