wpiutil: Replace LLVM Optional with C++17-compatible optional

Imported from https://github.com/akrzemi1/Optional with minor changes:
- Compiler conditional simplifications (we only use recent versions)
- Move from std::experimental to wpi namespace
- Change tests to integrate with Google Test

Update LLVM use cases.

ThirdPartyNotices.txt

@@ -31,6 +31,8 @@ sigslot               wpiutil/src/main/native/include/wpi/Signal.h
                       wpiutil/src/test/native/cpp/sigslot/
 tcpsockets            wpiutil/src/main/native/cpp/TCP{Stream,Connector,Acceptor}.cpp
                       wpiutil/src/main/native/include/wpi/TCP*.h
+Optional              wpiutil/src/main/native/include/wpi/optional.h
+                      wpiutil/src/test/native/cpp/test_optional.cpp
 
 
 ==============================================================================
@@ -212,3 +214,33 @@ distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
+
+
+==============================================================================
+Optional License
+==============================================================================
+Copyright (C) 2011 - 2017 Andrzej Krzemienski.
+
+Boost Software License - Version 1.0 - August 17th, 2003
+
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
+works are solely in the form of machine-executable object code generated by
+a source language processor.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
+FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.