[Support] [Debuginfod] Move HTTPClient to Debuginfod library.

Following the discussion in D112753, this moves the HTTPClient from Support to Debuginfod library so that tools depending on Support do not automatically depend on Curl as well. This also removes `HTTPClient::initialize()` and `HTTPClient::cleanup()` from `InitLLVM` so these steps should be implemented by user tools instead.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D115131