Annotate explicitly not used function as such.

These functions are explicitly there for a test scenario, so
should not be removed.

This keeps the clang-tidy [clang-diagnostic-unused-function] check
calm and relaxed about these.

#xls-build-gardener

PiperOrigin-RevId: 684112930

xls/common/casts_nc.cc

@@ -105,6 +105,7 @@ Derived3 WontCompile(Base& r) { return down_cast<Derived3>(r); }
 // Finally, tests that good code does compile (i.e. the compiler is
 // not blindly failing everything).
 
+// NOLINTNEXTLINE(clang-diagnostic-unused-function)
 static int Identity(int x) { return x; }
 
 #endif

xls/common/strong_int_nc.cc

@@ -27,6 +27,7 @@ class Explicit {
 };
 
 // Each case specified under each symbol must fail to compile.
+// NOLINTNEXTLINE(clang-diagnostic-unused-function)
 static void MustNotCompile() {
   USD dollars(1);
   (void)dollars;  // Avoid unused variable warning.