Fix FOLLY_STORAGE_CONSTEXPR for icc

Summary:
icc accepts the syntax, but doesn't emit the definition into the
object file.

Reviewed By: ricklavoie, yfeldblum

Differential Revision: D10228495

fbshipit-source-id: 73c1676deb4963a9c30886414cc1722d44a79214

folly/Portability.h

@@ -434,7 +434,11 @@ constexpr auto kCpplibVer = 0;
 #define FOLLY_STORAGE_CONSTEXPR
 #define FOLLY_STORAGE_CPP14_CONSTEXPR
 #else
+#if __ICC
+#define FOLLY_STORAGE_CONSTEXPR
+#else
 #define FOLLY_STORAGE_CONSTEXPR constexpr
+#endif
 #if FOLLY_USE_CPP14_CONSTEXPR
 #define FOLLY_STORAGE_CPP14_CONSTEXPR constexpr
 #else