Update LLVM to latest upstream. (#1080)

Also change header guards to WPI header guards.
Remove StringRef::c_str() customization, replacing the handful of uses with Twine or SmallString.
TCPStream: Include errno.h and make Windows includes lowercase for consistency.

Upstream LLVM version: eb4186cca7924fb1706357545311a2fa3de40c59

wpiutil/src/main/native/cpp/json.cpp

@@ -207,7 +207,7 @@ json::json(initializer_list_t init,
         std::for_each(init.begin(), init.end(), [this](const detail::json_ref<json>& element_ref)
         {
             auto element = element_ref.moved_or_copied();
-            m_value.object->emplace_second(
+            m_value.object->try_emplace(
                 *((*element.m_value.array)[0].m_value.string),
                 std::move((*element.m_value.array)[1]));
         });