[clangd] Reduce number of threads used by BackgroundIndex to number of physical cores.

[kadircet]

Summary

clangd is using as many threads as logical cores for BackgroundIndex
by default. We observed that it increases latency of foreground tasks.

This patch aims to change that default to number of physical cores to get rid of
that extra latency.

[ilya-biryukov]

LGTM.
NIT about a description: we can't be 100% certain that it's related to hyper-threading, so I'd avoid putting that to the description, maybe simply mention that this avoids higher latency for foreground operations?

clangd/index/Background.h

 // all commands in a compilation database. Indexing happens in the background.
 // FIXME: it should also persist its state on disk for fast start.
 // FIXME: it should watch for changes to files on disk.
 class BackgroundIndex : public SwapIndex {
 public:
   /// If BuildIndexPeriodMs is greater than 0, the symbol index will only be
   /// rebuilt periodically (one per \p BuildIndexPeriodMs); otherwise, index is
   /// rebuilt for each indexed file.
-  BackgroundIndex(Context BackgroundContext, const FileSystemProvider &,
-                  const GlobalCompilationDatabase &CDB,
-                  BackgroundIndexStorage::Factory IndexStorageFactory,
-                  size_t BuildIndexPeriodMs = 0,
-                  size_t ThreadPoolSize = llvm::hardware_concurrency());
+  BackgroundIndex(
+      Context BackgroundContext, const FileSystemProvider &,
+      size_t ThreadPoolSize = llvm::heavyweight_hardware_concurrency());
   ~BackgroundIndex(); // Blocks while the current task finishes.
 
   // Enqueue translation units for indexing.
   // The indexing happens in a background thread, so the symbols will be
   // available sometime later.
   void enqueue(const std::vector<std::string> &ChangedFiles);
 
   // Cause background threads to stop after ther current task, any remaining