Make llvm-as --help great again

This is a follow-up to https://reviews.llvm.org/D60411, but for llvm-as.

New output:

    OVERVIEW: llvm .ll -> .bc assembler

    USAGE: llvm-as [options] <input .llvm 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-as Options:

      -data-layout=<layout-string> - data layout string to use
      -disable-output              - Disable output
      -f                           - Enable binary output on terminals
      -module-hash                 - Emit module hash
      -o=<filename>                - Override output filename

Differential Revision: https://reviews.llvm.org/D60603

llvm-svn: 361750