[Support] [DebugInfo] Lazily create cache dir.

This change defers creating Support/Caching.cpp's cache directory until
it actually writes to the cache.

This allows using Caching library in a read-only fashion. If read-only,
the cache is guaranteed not to write to disk. This keeps tools using
DebugInfod (currently llvm-symbolizer) hermetic when not configured to
perform remote lookups.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D117589