[clangd][NFC] includes missing headers

[cjdb]

Summary

Shutdown.h was transitively depending on two headers, but this isn't
allowed under a modules build, so they're now explicitly included.

[sammccall]

Thank you!

clang-tools-extra/clangd/support/Shutdown.h

 //    returns an error, etc.
 //
 //===----------------------------------------------------------------------===//
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_SHUTDOWN_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_SHUTDOWN_H
 
 #include <cerrno>
 #include <chrono>
+#include <type_traits>
+#include <utility>
 
 namespace clang {
 namespace clangd {
 
 /// Causes this process to crash if still running after Timeout.
 void abortAfterTimeout(std::chrono::seconds Timeout);
 
 /// Sets a flag to indicate that clangd was sent a shutdown signal, and the