[AST] Add isOptional method to TypeBase

In `main` this method was added by 9ed732f. Since that commit is rather large, only cherry-pick the `isOptional` method to 5.5.

Author: ahoppen

include/swift/AST/Types.h

@@ -795,6 +795,9 @@ class alignas(1 << TypeAlignInBits) TypeBase {
   /// Check if this type is equal to Swift.Bool.
   bool isBool();
 
+  /// Check if this type is equal to Swift.Optional.
+  bool isOptional();
+
   /// Check if this type is equal to Builtin.IntN.
   bool isBuiltinIntegerType(unsigned bitWidth);
 

lib/AST/Type.cpp

@@ -628,6 +628,16 @@ bool TypeBase::isBool() {
   return false;
 }
 
+/// Check if this type is equal to Swift.Bool.
+bool TypeBase::isOptional() {
+  if (auto generic = getAnyGeneric()) {
+    if (isa<EnumDecl>(generic)) {
+      return getASTContext().getOptionalDecl() == generic;
+    }
+  }
+  return false;
+}
+
 Type TypeBase::getRValueType() {
   // If the type is not an lvalue, this is a no-op.
   if (!hasLValueType())