Switch std::call_once to llvm::call_once

Summary:
The std::call_once implementation in libstdc++ has problems on few systems: NetBSD, OpenBSD and Linux PPC. LLVM ships with a homegrown implementation llvm::call_once to help on these platforms.

This change is required in the NetBSD LLDB port. std::call_once with libstdc++ results with crashing the debugger.

Sponsored by <The NetBSD Foundation>

Reviewers: labath, joerg, emaste, mehdi_amini, clayborg

Reviewed By: labath, clayborg

Subscribers: #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D29288

llvm-svn: 294202

lldb/source/Utility/ConstString.cpp

@@ -20,6 +20,7 @@
 #include "llvm/Support/RWMutex.h"
 
 // Project includes
+#include "llvm/Support/Threading.h"
 #include "lldb/Utility/Stream.h"
 
 using namespace lldb_private;
@@ -191,10 +192,10 @@ protected:
 // touch ConstStrings is difficult.  So we leak the pool instead.
 //----------------------------------------------------------------------
 static Pool &StringPool() {
-  static std::once_flag g_pool_initialization_flag;
+  static llvm::once_flag g_pool_initialization_flag;
   static Pool *g_string_pool = nullptr;
 
-  std::call_once(g_pool_initialization_flag,
+  llvm::call_once(g_pool_initialization_flag,
                  []() { g_string_pool = new Pool(); });
 
   return *g_string_pool;