[clangd] Use compile-command interpolation to provide commands for header files.

[sammccall]

Summary

This uses the inferring wrapper introduced in D45006.

[sammccall]

Sending for review in case you have an opinion about whether it's worth plumbing a flag through here, or if it's fine to just always do it.

[ilya-biryukov]

Let's just always do it!

clang-tools-extra/trunk/clangd/GlobalCompilationDatabase.cpp

 tooling::CompilationDatabase *
 DirectoryBasedGlobalCompilationDatabase::getCDBInDirLocked(PathRef Dir) const {
   // FIXME(ibiryukov): Invalidate cached compilation databases on changes
   auto CachedIt = CompilationDatabases.find(Dir);
   if (CachedIt != CompilationDatabases.end())
     return CachedIt->second.get();
   std::string Error = "";
   auto CDB = tooling::CompilationDatabase::loadFromDirectory(Dir, Error);
+  if (CDB)
+    CDB = tooling::inferMissingCompileCommands(std::move(CDB));
   auto Result = CDB.get();
   CompilationDatabases.insert(std::make_pair(Dir, std::move(CDB)));
   return Result;
 }
 
 tooling::CompilationDatabase *
 DirectoryBasedGlobalCompilationDatabase::getCDBForFile(PathRef File) const {
   namespace path = llvm::sys::path;