[llvm] [Debuginfod] LLVM debuginfod server.

This implements a debuginfod server in llvm using the `DebuginfodCollection` and `DebuginfodServer` classes. This is tested with lit tests against the debuginfod-find client.

The server scans 0 or more local directories for artifacts. It serves the debuginfod protocol over HTTP. Only the `executable` and `debuginfo` endpoints are supported (no `/source` endpoint).
The server also uses the debuginfod client as a fallback, so it can hit the local debuginfod cache or federate to other known debuginfod servers.
The client behavior is controllable through the standard environment variables (`DEBUGINFOD_URLS`, `DEBUGINFOD_CACHE_PATH`, `DEBUGINFOD_TIMEOUT`)

The server implements on-demand collection updates as follows:
If the build-id is not found by a local lookup, rescan immediately and look up the build-id again before returning 404. To protect against DoS attacks, do not rescan more frequently than once per N seconds (specified by `-m`).

Lit tests are provided which test the `llvm-debuginfod-find` client against the `llvm-debuginfod` server.

Reviewed By: mysterymath

Differential Revision: https://reviews.llvm.org/D114846

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