Update LLVM and switch to std::optional. (#2424)

LLVM's bazel build has changed a bit, so this updates the tree for that.

LLVM is also moving `llvm::Optional` to match the standard API, but it seemed simpler to just switch to `std::optional`.

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/source/source_buffer.cpp

@@ -12,12 +12,12 @@
 #include <cerrno>
 #include <cstdint>
 #include <limits>
+#include <optional>
 #include <system_error>
 #include <utility>
 #include <variant>
 
 #include "common/check.h"
-#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/Support/Error.h"