[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/SBVariablesOptions.cpp

@@ -97,7 +97,7 @@ operator=(const SBVariablesOptions &options) {
       SBVariablesOptions, operator=,(const lldb::SBVariablesOptions &),
       options);
 
-  m_opaque_up.reset(new VariablesOptionsImpl(options.ref()));
+  m_opaque_up = std::make_unique<VariablesOptionsImpl>(options.ref());
   return LLDB_RECORD_RESULT(*this);
 }