support clang compiler

Summary: `clang-cl` and some other invocations of `clang` on windows define `_MSC_VER`. However `clang` does not implement `#pragma optimize` so we need to use the other code-path here.

Reviewed By: yfeldblum

Differential Revision: D22301661

fbshipit-source-id: e51656497a5fc109bf04f7daac82f824ba8c5c8e

folly/Benchmark.h

@@ -278,7 +278,7 @@ addBenchmark(const char* file, StringPiece name, Lambda&& lambda) {
  * benchmark but not in real use cases.
  */
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 
 #pragma optimize("", off)