[Debuginfod] Don't depend on Content-Length.

The present implementation of debuginfod lookups requires the
Content-Length field to be populated in the HTTP server response.
Unfortunately, Content-Length is optional, and there are some real
scenarios where it's missing. (For example, a Google Cloud Storage
server doing on-the-fly gunzipping.)

This changes the debuginfod response handler to directly stream the
output to the cache file as it is received. In addition to allowing
lookups to proceed without a Content-Lenght, it seems somewhat more
straightforward to implement, and it allows the disk I/O to be
interleaved with the network I/O.

Reviewed By: noajshu

Differential Revision: https://reviews.llvm.org/D121720