[clangd] Attempt to fix buildbots

http://45.33.8.238/win/47615/step_4.txt is a sample error; I believe it
just needs the right header to be included.

Author: smeenai

clang-tools-extra/clangd/support/ThreadCrashReporter.cpp

@@ -9,6 +9,7 @@
 #include "support/ThreadCrashReporter.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/ThreadLocal.h"
+#include <atomic>
 
 namespace clang {
 namespace clangd {