[DebugInfod] Prefix debuginfod cache directory

This prevents the debuginfod client from dumping files directly into the
default cache directory (e.g., ~/.cache). Instead, these files are
placed in a subdirectory (e.g., ~/.cache/llvm-debuginfod/client).
Behavior is unaffected if the cache directory is provided by the
DEBUGINFO_CACHE_PATH environment variable.

Patch By: mysterymath

Differential Revision: https://reviews.llvm.org/D117619