[clang] Fix -Wunused-variable in ByteCodeStmtGen.cpp (NFC)

llvm-project/clang/lib/AST/Interp/ByteCodeStmtGen.cpp:196:31:
error: unused variable 'B' [-Werror,-Wunused-variable]
          const Record::Base *B = R->getVirtualBase(BaseDecl);
                              ^
1 error generated.