Add missing new method to clang::serialization::DataStreamBasic{Reader,Writer} subclasses

See https://github.com/llvm/llvm-project/pull/134142

include/swift/ClangImporter/SwiftAbstractBasicReader.h

@@ -64,6 +64,11 @@ public:
     return uint32_t(asImpl().readUInt64());
   }
 
+  clang::UnsignedOrNone readUnsignedOrNone() {
+    return clang::UnsignedOrNone::fromInternalRepresentation(
+        unsigned(asImpl().readUInt64()));
+  }
+
   clang::Selector readSelector() {
     uint64_t numArgsPlusOne = asImpl().readUInt64();