[clangd] dexp tool uses llvm::cl to parse its flags.

Summary:
We can use cl::ResetCommandLineParser() to support different types of
command-lines, as long as we're careful about option lifetimes.
(I tried using subcommands, but the error messages were bad)
I found a mostly-reasonable pattern to isolate the fiddly parts.

Added -scope and -limit flags to the `find` command to demonstrate.
(Note that scope support seems to be broken in dex?)

Fixed symbol lookup to parse symbol IDs.

Caveats:
 - with command help (e.g. `find -help`), you also get some spam
   about required arguments. This is a bug in llvm::cl, which prints
   these to errs() rather than the designated stream.

Reviewers: kbobyrev

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D51989

llvm-svn: 342456