[StaticAnalyzer] Fix -Wunused-variable in SVals.h (NFC)

/llvm-project/clang/include/clang/StaticAnalyzer/Core/PathSensitive/SVals.h:321:14: error: unused variable 'K' [-Werror,-Wunused-variable]
    SValKind K = data.first.getKind();
             ^
1 error generated.