[Support] lazy initialize OutputMutex in CodeGenCoverage

[ychen]

Summary

It seems this is desired for libSupport otherwise it trips
'-Werror=global-constructors'.

llvm/lib/Support/CodeGenCoverage.cpp

 //===- lib/Support/CodeGenCoverage.cpp -------------------------------------==//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 /// \file
 /// This file implements the CodeGenCoverage class.
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/CodeGenCoverage.h"
 
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Mutex.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/ScopedPrinter.h"
 #include "llvm/Support/ToolOutputFile.h"
 
 using namespace llvm;
 
-static sys::SmartMutex<true> OutputMutex;
+static ManagedStatic<sys::SmartMutex<true>> OutputMutex;
 
 CodeGenCoverage::CodeGenCoverage() {}
 
 void CodeGenCoverage::setCovered(uint64_t RuleID) {
   if (RuleCoverage.size() <= RuleID)
     RuleCoverage.resize(RuleID + 1, 0);
   RuleCoverage[RuleID] = true;
 }
bool CodeGenCoverage::parse(MemoryBuffer &Buffer, StringRef BackendName) {
   }
 
   return true;
 }
 
 bool CodeGenCoverage::emit(StringRef CoveragePrefix,
                            StringRef BackendName) const {
   if (!CoveragePrefix.empty() && !RuleCoverage.empty()) {
-    sys::SmartScopedLock<true> Lock(OutputMutex);
+    sys::SmartScopedLock<true> Lock(*OutputMutex);
 
     // We can handle locking within a process easily enough but we don't want to
     // manage it between multiple processes. Use the process ID to ensure no
     // more than one process is ever writing to the same file at the same time.
     std::string Pid = llvm::to_string(sys::Process::getProcessId());
 
     std::string CoverageFilename = (CoveragePrefix + Pid).str();