[llvm] [Debuginfo] Debuginfod client library.

This adds a Debuginfod library containing the `fetchDebuginfo` function which queries servers specified by the `DEBUGINFOD_URLS` environment variable for the debuginfo, executable, or a specified source file associated with a given build id.

This diff was split out from D111252.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D112758