Revert: [clangd] Replace an include with a forward declaration (llvm#97082)

Reverting due to failures on several buildbots.

Author: doru1004

clang-tools-extra/clangd/index/remote/Client.h

@@ -9,15 +9,14 @@
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_INDEX_REMOTE_CLIENT_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_INDEX_REMOTE_CLIENT_H
 
+#include "index/Index.h"
 #include "llvm/ADT/StringRef.h"
 
 #include <memory>
 
 namespace clang {
 namespace clangd {
 
-class SymbolIndex;
-
 namespace remote {
 
 /// Returns an SymbolIndex client that passes requests to remote index located