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.

wpiutil/src/main/native/include/wpi/STLExtras.h

@@ -17,10 +17,10 @@
 #ifndef WPIUTIL_WPI_STLEXTRAS_H
 #define WPIUTIL_WPI_STLEXTRAS_H
 
-#include "wpi/LLVMOptional.h"
 #include "wpi/SmallVector.h"
 #include "wpi/iterator.h"
 #include "wpi/iterator_range.h"
+#include "wpi/optional.h"
 #include <algorithm>
 #include <cassert>
 #include <cstddef>