[Debuginfod] Add --debug-file-directory to llvm-debuginfod-find.

This allows llvm-debuginfod-find to locate binaries in local build ID
directories configured via --debug-file-directory, the same flag used
for this purpose by llvm-symbolizer. This provides a consistent lookup
semantics between the two tools when configured the same way, in
particular when debug binaries may be located either locally or
remotely.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D125864