Fix spurious warning 'misc-use-anonymous-namespace' (#1860) (#1861)

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

diff --git a/include/benchmark/benchmark.h b/include/benchmark/benchmark.h
index 53a2224..86f9dbb 100644
--- a/include/benchmark/benchmark.h
+++ b/include/benchmark/benchmark.h
@@ -1554,6 +1554,7 @@
   BaseClass##_##Method##_Benchmark
 
 #define BENCHMARK_PRIVATE_DECLARE(n)                                 \
+  /* NOLINTNEXTLINE(misc-use-anonymous-namespace) */                 \
   static ::benchmark::internal::Benchmark* BENCHMARK_PRIVATE_NAME(n) \
       BENCHMARK_UNUSED