benchmark

Fix -Wdeprecated-declarations warning triggered by clang-cl. (#1245)

WebRTC uses Google Benchmarks as a dependency and uses Chromium's build
infrastructure. Chromium is compiled using clang-cl on Windows, and the
-Wdeprecated-declarations warning is triggered. Because clang-cl accepts
gcc's diagnostic prama and defines the __clang__ macro,
using it can solve this issue.

Bug: webrtc:13280