[clangd][Background] Make index validation logs verbose

[kadircet]

Summary

These errors are non-harmful and should be transient. They either
imply:

• compilation database returned stale results for TUs and it'll be fixed once it's updated to match project state.
• a TUs dependencies has changed and some headers no longer exist. this should be fixed with the next indexing cycle.

In either case the user will have some stale symbols in their index until clangd
restarts and the underlying issue is resolved. On the downside these logs are
confusing users when there's another issue.

clang-tools-extra/clangd/index/Background.cpp

if (llvm::sys::path::is_absolute(Cmd.Filename)) {
     llvm::sys::path::remove_dots(AbsolutePath, true);
   }
   return AbsolutePath;
 }
 
 bool shardIsStale(const LoadedShard &LS, llvm::vfs::FileSystem *FS) {
   auto Buf = FS->getBufferForFile(LS.AbsolutePath);
   if (!Buf) {
-    elog("Background-index: Couldn't read {0} to validate stored index: {1}",
+    vlog("Background-index: Couldn't read {0} to validate stored index: {1}",
          LS.AbsolutePath, Buf.getError().message());
     // There is no point in indexing an unreadable file.
     return false;
   }
   return digest(Buf->get()->getBuffer()) != LS.Digest;
 }
 
 } // namespace