[MCParser] Use std::optional in WasmAsmParser.cpp (NFC)

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

llvm/lib/MC/MCParser/WasmAsmParser.cpp

@@ -26,6 +26,7 @@
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSymbolWasm.h"
 #include "llvm/Support/Casting.h"
+#include <optional>
 
 using namespace llvm;
 
@@ -152,7 +153,7 @@ public:
     if (Lexer->isNot(AsmToken::String))
       return error("expected string in directive, instead got: ", Lexer->getTok());
 
-    auto Kind = StringSwitch<Optional<SectionKind>>(Name)
+    auto Kind = StringSwitch<std::optional<SectionKind>>(Name)
                     .StartsWith(".data", SectionKind::getData())
                     .StartsWith(".tdata", SectionKind::getThreadData())
                     .StartsWith(".tbss", SectionKind::getThreadBSS())