[c-interop] Use llvm::Optional instead of std::optional for now

The Attr.h is shared with SwiftCompilerSources through C++ interop and
C++ interop somehow crashes with libc++'s std::optional. So use legacy
llvm::Optional for now.

Author: kateinoigakukun

include/swift/AST/Attr.h

@@ -2343,25 +2343,25 @@ class ExposeAttr : public DeclAttribute {
 /// the specified way to interoperate with Swift.
 class ExternAttr : public DeclAttribute {
 public:
-  ExternAttr(std::optional<StringRef> ModuleName, std::optional<StringRef> Name,
+  ExternAttr(llvm::Optional<StringRef> ModuleName, llvm::Optional<StringRef> Name,
              SourceLoc AtLoc, SourceRange Range, ExternKind Kind, bool Implicit)
       : DeclAttribute(DAK_Extern, AtLoc, Range, Implicit),
         ModuleName(ModuleName), Name(Name) {
     Bits.ExternAttr.kind = static_cast<unsigned>(Kind);
   }
 
-  ExternAttr(std::optional<StringRef> ModuleName, std::optional<StringRef> Name,
+  ExternAttr(llvm::Optional<StringRef> ModuleName, llvm::Optional<StringRef> Name,
              ExternKind Kind, bool Implicit)
       : ExternAttr(ModuleName, Name, SourceLoc(), SourceRange(), Kind,
                    Implicit) {}
 
   /// The module name to import the named declaration in it
   /// Used for Wasm import declaration.
-  const std::optional<StringRef> ModuleName;
+  const llvm::Optional<StringRef> ModuleName;
 
   /// The declaration name to import
   /// std::nullopt if the declaration name is not specified with @_extern(c)
-  const std::optional<StringRef> Name;
+  const llvm::Optional<StringRef> Name;
 
   /// Returns the kind of extern.
   ExternKind getExternKind() const {

lib/Parse/ParseDecl.cpp

@@ -1450,7 +1450,7 @@ bool Parser::parseExternAttribute(DeclAttributes &Attributes,
   }
 
   auto parseStringLiteralArgument =
-      [&](std::optional<StringRef> fieldName) -> std::optional<StringRef> {
+      [&](std::optional<StringRef> fieldName) -> llvm::Optional<StringRef> {
     if (!consumeIf(tok::comma)) {
       diagnose(Loc, diag::attr_expected_comma, AttrName,
                DeclAttribute::isDeclModifier(DAK_Extern));
@@ -1489,7 +1489,7 @@ bool Parser::parseExternAttribute(DeclAttributes &Attributes,
 
   // Parse @_extern(wasm, module: "x", name: "y") or @_extern(c[, "x"])
   ExternKind kind;
-  std::optional<StringRef> importModuleName, importName;
+  llvm::Optional<StringRef> importModuleName, importName;
 
   if (kindTok.getText() == "wasm") {
     kind = ExternKind::Wasm;

lib/Serialization/Deserialization.cpp

@@ -5864,7 +5864,7 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
             scratch, isImplicit, rawKind, moduleNameSize, declNameSize);
 
         ExternKind kind = (ExternKind)rawKind;
-        std::optional<StringRef> moduleName, declName;
+        llvm::Optional<StringRef> moduleName, declName;
 
         switch (kind) {
         case ExternKind::C: {