[fidlgen] [llcpp] Design a more flexible set of flags for fidlgen_llcpp

The new flags are:

    --json: JSON IR path
    --header: Path where fidlgen_llcpp will write the generated header
    --source: Path where fidlgen_llcpp will write the generated .cc source
    --include-base: Directory relative to which include paths will be
    computed. This should be a parent directory of "header".

Change-Id: I42696ee51c98532f012134d213e38f412241b997