[gn build] sort of merge 37ac559

It'd be nicer if there was a group target that forwarded either to
//clang-tools-extra/clangd/index/remote or
//clangd/index/remote/unimplemented based on if remote index is enabled,
but for now it's never enabled in the gn build.

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

@@ -6,6 +6,7 @@ executable("clangd") {
     "//clang-tools-extra/clang-tidy",
     "//clang-tools-extra/clangd",
     "//clang-tools-extra/clangd:features",
+    "//clang-tools-extra/clangd/index/remote/unimplemented",
     "//clang-tools-extra/clangd/refactor/tweaks",
     "//clang/lib/AST",
     "//clang/lib/Basic",