Sanitize llvm-extract -help output

Filter out irrelevant options

New output:

    OVERVIEW: llvm extractor

    USAGE: llvm-extract [options] <input bitcode file>

    OPTIONS:

    Generic Options:

      --help              - Display available options (--help-hidden for more)
      --help-list         - Display list of available options (--help-list-hidden for more)
      --version           - Display the version of this program

    llvm-extract Options:

      --alias=<alias>     - Specify alias to extract
      --bb=<function:bb>  - Specify <function, basic block> pairs to extract
      --delete            - Delete specified Globals from Module
      -f                  - Enable binary output on terminals
      --func=<function>   - Specify function to extract
      --glob=<global>     - Specify global to extract
      -o=<filename>       - Specify output filename
      --ralias=<ralias>   - Specify alias(es) to extract using a regular expression
      --recursive         - Recursively extract all called functions
      --rfunc=<rfunction> - Specify function(s) to extract using a regular expression
      --rglob=<rglobal>   - Specify global(s) to extract using a regular expression

Differential Revision: https://reviews.llvm.org/D62511

llvm-svn: 363201