Add debuginfod client in lldb

[noajshu]

The debuginfod client in llvm is now integrated in two llvm tools llvm-debuginfod-find and llvm-symbolizer. lldb is a natural next target to integrate the client library.

[fche]

cc: @kwk fyi

[russelltg]

I'm interested in working on this--let me know if someone is already working on this as to not duplicate work

[russelltg]

I'm thinking it likely makes sense to tackle this in 3 separate changesets: source file, debug info, and executable fetching from debuginfod.

[russelltg]

I think this is added as of lldb 18.

It still can't fetch sources, but can fetch binaries and debuginfo.