[lldb] Add image dump pcm-info command

Add `pcm-info` to the `target module dump` subcommands.

This dump command shows information about clang .pcm files. This command
effectively runs `clang -module-file-info` and produces identical output.

The .pcm file format is tightly coupled to the clang version. The clang
embedded in lldb is not guaranteed to match the version of the clang executable
available on the local system.

There have been times when I've needed to view the details about a .pcm file
produced by lldb's embedded clang, but because the clang executable was a
slightly different version, the `-module-file-info` invocation failed. With
this command, users can inspect .pcm files generated by lldb too.

Differential Revision: https://reviews.llvm.org/D129456