[llvm] [Debuginfod] DebuginfodCollection and DebuginfodServer for tracking local debuginfo.

This library implements the class `DebuginfodCollection`, which scans a set of directories for binaries, classifying them according to whether they contain debuginfo. This also provides the `DebuginfodServer`, an `HTTPServer` which serves debuginfod's `/debuginfo` and `/executable` endpoints. This is intended as the final new supporting library required for `llvm-debuginfod`.

As implemented here, `DebuginfodCollection` only finds ELF binaries and DWARF debuginfo. All other files are ignored. However, the class interface is format-agnostic. Generalizing to support other platforms will require refactoring of LLVM's object parsing libraries to eliminate use of `report_fatal_error` ([[ https://github.com/llvm/llvm-project/blob/main/llvm/lib/Object/WasmObjectFile.cpp#L74 | e.g. when reading WASM files ]]), so that the debuginfod daemon does not crash when it encounters a malformed file on the disk.

The `DebuginfodCollection` is tested by end-to-end tests of the debuginfod server (D114846).

Reviewed By: mysterymath

Differential Revision: https://reviews.llvm.org/D114845