Disable the constexpr tests for findFirstSet, findLastSet and nextPowTwo under MSVC

Summary: Because they aren't currently constexpr under MSVC.

Reviewed By: yfeldblum

Differential Revision: D3705095

fbshipit-source-id: ea266ebd9677fb2ba232476160f7c10a23954db9

folly/test/BitsTest.cpp

@@ -23,12 +23,15 @@
 using namespace folly;
 
 // Test constexpr-ness.
-#ifndef __clang__
+#if !defined(__clang__) && !defined(_MSC_VER)
 static_assert(findFirstSet(2u) == 2, "findFirstSet");
 static_assert(findLastSet(2u) == 2, "findLastSet");
 static_assert(nextPowTwo(2u) == 2, "nextPowTwo");
+#endif
+
+#ifndef __clang__
 static_assert(isPowTwo(2u), "isPowTwo");
-#endif  // __clang__
+#endif
 
 namespace {