Include <cstdlib> for std::abort() in clangd

This fixes a "not a member of 'std'" error with e.g. Fedora 32.

Closes: #105
(cherry picked from commit 58592f6)

Author: DimitryAndric

clang-tools-extra/clangd/Shutdown.cpp

@@ -9,6 +9,7 @@
 #include "Shutdown.h"
 
 #include <atomic>
+#include <cstdlib>
 #include <thread>
 
 namespace clang {