Fix getBitWidthValue compilation error on LLVM 20+ (#1248)

* Fixes #1242.

Introduced by upstream in
llvm/llvm-project#122289

Ref: iovisor/bcc#5194

Author: kkysen

c2rust-ast-exporter/src/AstExporter.cpp

@@ -2213,8 +2213,12 @@ class TranslateASTVisitor final
 
                          // 2. Encode bitfield width if any
                          if (D->isBitField()) {
-                             cbor_encode_uint(
-                                 array, D->getBitWidthValue(*this->Context));
+#if LLVM_VERSION_MAJOR >= 20
+                             const auto bitWidthValue = D->getBitWidthValue();
+#else
+                             const auto bitWidthValue = D->getBitWidthValue(*this->Context);
+#endif
+                             cbor_encode_uint(array, bitWidthValue);
                          } else {
                              cbor_encode_null(array);
                          };