[lldb/API] Use std::make_unique<> (NFC)

I was holding off on this change until we moved to C++14 as to not have
to convert llvm::make_unique to std::make_unique. That happened a while
ago so here's the first patch for the API which had a bunch of raw
`new`s.

lldb/source/API/SBStream.cpp

@@ -177,7 +177,7 @@ lldb_private::Stream *SBStream::get() { return m_opaque_up.get(); }
 
 lldb_private::Stream &SBStream::ref() {
   if (m_opaque_up == nullptr)
-    m_opaque_up.reset(new StreamString());
+    m_opaque_up = std::make_unique<StreamString>();
   return *m_opaque_up;
 }