Use the multi-lockable form of std::lock for operator=

For = operators for lists that have mutexes, we were either
just taking the locks sequentially or hand-rolling a trick
to try to avoid lock inversion.  Use the std::lock mechanism
for this instead.

Differential Revision: https://reviews.llvm.org/D59957

llvm-svn: 357276
diff --git a/lldb/source/Target/ThreadList.cpp b/lldb/source/Target/ThreadList.cpp
index 86248da..afdfda3 100644
--- a/lldb/source/Target/ThreadList.cpp
+++ b/lldb/source/Target/ThreadList.cpp
@@ -37,8 +37,10 @@
   if (this != &rhs) {
     // Lock both mutexes to make sure neither side changes anyone on us while
     // the assignment occurs
-    std::lock_guard<std::recursive_mutex> guard(GetMutex());
-    std::lock_guard<std::recursive_mutex> rhs_guard(rhs.GetMutex());
+    std::lock(GetMutex(), rhs.GetMutex());
+    std::lock_guard<std::recursive_mutex> guard(GetMutex(), std::adopt_lock);
+    std::lock_guard<std::recursive_mutex> rhs_guard(rhs.GetMutex(), 
+                                                    std::adopt_lock);
 
     m_process = rhs.m_process;
     m_stop_id = rhs.m_stop_id;