Added missing includes in clangd to fix the build.

This commit should fix buildbot failures.

llvm-svn: 303177
diff --git a/clang-tools-extra/clangd/JSONRPCDispatcher.cpp b/clang-tools-extra/clangd/JSONRPCDispatcher.cpp
index be6be77..5deb75a 100644
--- a/clang-tools-extra/clangd/JSONRPCDispatcher.cpp
+++ b/clang-tools-extra/clangd/JSONRPCDispatcher.cpp
@@ -12,6 +12,8 @@
 #include "llvm/ADT/SmallString.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/YAMLParser.h"
+#include <istream>
+
 using namespace clang;
 using namespace clangd;