[Debuginfod] DEBUGINFOD_HEADERS_FILE environment variable

This change adds a DEBUGINFOD_HEADERS_FILE environment variable provides
a file containing HTTP headers to attach to outgoing HTTP requests, one
per line. This allows a file permissioned with OS access control
mechanisms to supply bearer credentials for Debuginfod requests.

This matches the mechanism recently added to elfutils' libdebuginfod.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D136303