[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/SBError.cpp

@@ -149,7 +149,7 @@ SBError::operator bool() const {
 
 void SBError::CreateIfNeeded() {
   if (m_opaque_up == nullptr)
-    m_opaque_up.reset(new Status());
+    m_opaque_up = std::make_unique<Status>();
 }
 
 lldb_private::Status *SBError::operator->() { return m_opaque_up.get(); }