ClangLinkerWrapper: explicitly #include <atomic>

[nhaehnle]

Summary

This code relied on implicitly having std::atomic available via the
ManagedStatic.h header.

[efriedma]

LGTM

You don't really need pre-commit review for a patch that just adds an include, though.

clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp

 #include "llvm/Support/Program.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/StringSaver.h"
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Support/WithColor.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Target/TargetMachine.h"
+#include <atomic>
 
 using namespace llvm;
 using namespace llvm::opt;
 using namespace llvm::object;
 
 /// Path of the current binary.
 static const char *LinkerExecutable;