[Debuginfod] Flag-determine debuginfod lookups in llvm-symbolizer.

This change adds a pair of flags controlling whether llvm-symbolizer
attempts debuginfod lookups. Lookups are attempted if --debuginfod is
passed and disabled if --no-debuginfod is passed.

The default behavior is made more nuanced: debuginfod lookups are now
only attempted if an HTTP client is compiled in and at least one backing
debuginfod URL was configured via environment variable. Previously,
debuginfod lookups would always be attempted, even if there were no
chance that they could succeed.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D118665