gn build: Merge r366458.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366487 91177308-0d34-0410-b5e6-96231b3b80d8

utils/gn/secondary/clang-tools-extra/clangd/BUILD.gn

@@ -105,6 +105,7 @@ static_library("clangd") {
     "URI.cpp",
     "XRefs.cpp",
     "index/Background.cpp",
+    "index/BackgroundIndexLoader.cpp",
     "index/BackgroundIndexStorage.cpp",
     "index/BackgroundQueue.cpp",
     "index/BackgroundRebuild.cpp",