[C++11] Make use of 'nullptr' in the Support library.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205697 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Mutex.cpp

@@ -42,7 +42,7 @@ using namespace sys;
 
 // Construct a Mutex using pthread calls
 MutexImpl::MutexImpl( bool recursive)
-  : data_(0)
+  : data_(nullptr)
 {
   // Declare the pthread_mutex data structures
   pthread_mutex_t* mutex =