Fix spurious warning 'misc-use-anonymous-namespace' (google#1860)

Disables 'misc-use-anonymous-namespace' for usage of the BENCHMARK
macro. This warning is spurious, and the variable declared by the
BENCHMARK macro can't be moved into an annonymous namespace.

We don't want to disable it globally, but it can be disabled locally,
for the `BENCHMARK` statement, as this warning appears downstream for
users.

See:
https://clang.llvm.org/extra/clang-tidy/#suppressing-undesired-diagnostics

include/benchmark/benchmark.h

@@ -1554,6 +1554,7 @@ class Fixture : public internal::Benchmark {
   BaseClass##_##Method##_Benchmark
 
 #define BENCHMARK_PRIVATE_DECLARE(n)                                 \
+  /* NOLINTNEXTLINE(misc-use-anonymous-namespace) */                 \
   static ::benchmark::internal::Benchmark* BENCHMARK_PRIVATE_NAME(n) \
       BENCHMARK_UNUSED