Serialize clang_createIndex to prevent race condition (concurrent modify/read to FirstTarget defined in lib/Support/TargetRegistry.cpp)

[MaskRay]

Hopefully fixes #43

RegisterTarget modifies a global variable FirstTarget which may be concurrently read by other threads that are calling clang_createIndex.

(gdb) bt                    
#0  0x00007fd5ac25d280 in llvm::TargetRegistry::RegisterTarget(llvm::Target&, char const*, char const*, bool (*)(llvm::Triple::ArchType), bool) ()
   from /home/maskray/Dev/Util/cquery/build/clang+llvm-5.0.0-linux-x86_64-ubuntu14.04/lib/libclang.so.5
#1  0x00007fd5ab8fd5d8 in LLVMInitializeAArch64TargetInfo ()                                                      
   from /home/maskray/Dev/Util/cquery/build/clang+llvm-5.0.0-linux-x86_64-ubuntu14.04/lib/libclang.so.5
#2  0x00007fd5aae90a86 in llvm::InitializeAllTargets() ()                                                         
   from /home/maskray/Dev/Util/cquery/build/clang+llvm-5.0.0-linux-x86_64-ubuntu14.04/lib/libclang.so.5
#3  0x00007fd5aae908de in clang_createIndex ()           
   from /home/maskray/Dev/Util/cquery/build/clang+llvm-5.0.0-linux-x86_64-ubuntu14.04/lib/libclang.so.5
#4  0x000055bd41986583 in ClangIndex::ClangIndex (this=0x7fd5a27fbd78, exclude_declarations_from_pch=1, 
    display_diagnostics=0) at ../src/clang_index.cc:8    
#5  0x000055bd4198655e in ClangIndex::ClangIndex (this=0x7fd5a27fbd78) at ../src/clang_index.cc:3
#6  0x000055bd4198d31a in IndexMain (config=0x55bd4333ea90, file_consumer_shared=0x7ffeaa7e75b0, 
    timestamp_manager=0x7ffeaa7e71f0, import_manager=0x7ffeaa7e7250, project=0x7ffeaa7e7670, 
    working_files=0x7ffeaa7e7610, waiter=0x7ffeaa7e7f10, queue=0x7ffeaa7e7a40) at ../src/command_line.cc:509
#7  0x000055bd41a4864c in (anonymous namespace)::InitializeHandler::<lambda()>::operator()(void) const (
    __closure=0x55bd43341ac8) at ../src/messages/initialize.cc:175                                                
#8  0x000055bd41a4a7bb in std::_Function_handler<WorkThread::Result(), (anonymous namespace)::InitializeHandler::Run((anonymous namespace)::Ipc_InitializeRequest*)::<lambda()> >::_M_invoke(const std::_Any_data &) (
    __functor=...) at /usr/include/c++/6/functional:1717 
#9  0x000055bd41b4ec12 in std::function<WorkThread::Result ()>::operator()() const (this=0x55bd43341ac8)
    at /usr/include/c++/6/functional:2127                
#10 0x000055bd41b4e4ee in WorkThread::<lambda()>::operator()(void) const (__closure=0x55bd43341aa8)
    at ../src/work_thread.cc:18                          
#11 0x000055bd41b4eb78 in std::_Bind_simple<WorkThread::StartThread(const string&, const std::function<WorkThread::Result()>&)::<lambda()>()>::_M_invoke<>(std::_Index_tuple<>) (this=0x55bd43341aa8)
    at /usr/include/c++/6/functional:1391                
#12 0x000055bd41b4eb15 in std::_Bind_simple<WorkThread::StartThread(const string&, const std::function<WorkThread::Result()>&)::<lambda()>()>::operator()(void) (this=0x55bd43341aa8) at /usr/include/c++/6/functional:1380
#13 0x000055bd41b4eaf4 in std::thread::_State_impl<std::_Bind_simple<WorkThread::StartThread(const string&, const std::function<WorkThread::Result()>&)::<lambda()>()> >::_M_run(void) (this=0x55bd43341aa0)
    at /usr/include/c++/6/thread:197                     
#14 0x00007fd5aa771e6f in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6

[jacobdufault]

Hope it fixes it, code LG