[DebugInfod] Prefix debuginfod cache directory.

[mysterymath]

Summary

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.

[phosek]

LGTM

[mysterymath]

Note: I don't yet have commit access; I'll shoot of an email about it later this afternoon.

[phosek]

In D117619#3253032, @mysterymath wrote:

Note: I don't yet have commit access; I'll shoot of an email about it later this afternoon.

No problem, I can land this for you.

llvm/lib/Debuginfod/Debuginfod.cpp

 Expected<std::string> getDefaultDebuginfodCacheDirectory() {
   if (const char *CacheDirectoryEnv = std::getenv("DEBUGINFOD_CACHE_PATH"))
     return CacheDirectoryEnv;
 
   SmallString<64> CacheDirectory;
   if (!sys::path::cache_directory(CacheDirectory))
     return createStringError(
         errc::io_error, "Unable to determine appropriate cache directory.");
+  sys::path::append(CacheDirectory, "llvm-debuginfod", "client");
   return std::string(CacheDirectory);
 }
 
 std::chrono::milliseconds getDefaultDebuginfodTimeout() {
   long Timeout;
   const char *DebuginfodTimeoutEnv = std::getenv("DEBUGINFOD_TIMEOUT");
   if (DebuginfodTimeoutEnv &&
       to_integer(StringRef(DebuginfodTimeoutEnv).trim(), Timeout, 10))