[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.

clang/include/clang/StaticAnalyzer/Core/PathSensitive/SVals.h

@@ -318,7 +318,7 @@ class LocAsInteger : public NonLoc {
       : NonLoc(LocAsIntegerKind, &data) {
     // We do not need to represent loc::ConcreteInt as LocAsInteger,
     // as it'd collapse into a nonloc::ConcreteInt instead.
-    SValKind K = data.first.getKind();
+    [[maybe_unused]] SValKind K = data.first.getKind();
     assert(K == loc::MemRegionValKind || K == loc::GotoLabelKind);
   }