Hack around MSVC not supporting constexpr storage

Summary:
[Folly] Hack around MSVC not supporting `constexpr` storage.

GCC and Clang do, but MSVC does not, support:

```lang=c++
extern int const num;
constexpr int const num = 3;
```

Work around it.

Reviewed By: elsteveogrande

Differential Revision: D6903413

fbshipit-source-id: 3a0957816d8d82718fb6187dc1803df1605f80aa

folly/Format.cpp

@@ -75,17 +75,17 @@ struct format_table_sign_make_item {
 };
 
 //  the tables
-constexpr auto formatAlignTable =
+FOLLY_STORAGE_CONSTEXPR auto formatAlignTable =
     make_array_with<256>(format_table_align_make_item{});
-constexpr auto formatSignTable =
+FOLLY_STORAGE_CONSTEXPR auto formatSignTable =
     make_array_with<256>(format_table_sign_make_item{});
-constexpr decltype(formatHexLower) formatHexLower =
+FOLLY_STORAGE_CONSTEXPR decltype(formatHexLower) formatHexLower =
     make_array_with<256>(format_table_conv_make_item<16, 2, false>{});
-constexpr decltype(formatHexUpper) formatHexUpper =
+FOLLY_STORAGE_CONSTEXPR decltype(formatHexUpper) formatHexUpper =
     make_array_with<256>(format_table_conv_make_item<16, 2, true>{});
-constexpr decltype(formatOctal) formatOctal =
+FOLLY_STORAGE_CONSTEXPR decltype(formatOctal) formatOctal =
     make_array_with<512>(format_table_conv_make_item<8, 3>{});
-constexpr decltype(formatBinary) formatBinary =
+FOLLY_STORAGE_CONSTEXPR decltype(formatBinary) formatBinary =
     make_array_with<256>(format_table_conv_make_item<2, 8>{});
 
 } // namespace detail

folly/GroupVarint.cpp

@@ -131,11 +131,12 @@ struct group_varint_table_sse_mask_make_item
 };
 
 #if FOLLY_SSE >= 3
-alignas(16) constexpr decltype(groupVarintSSEMasks) groupVarintSSEMasks =
+alignas(16) FOLLY_STORAGE_CONSTEXPR
+    decltype(groupVarintSSEMasks) groupVarintSSEMasks =
         make_array_with<256>(group_varint_table_sse_mask_make_item{});
 #endif
 
-constexpr decltype(groupVarintLengths) groupVarintLengths =
+FOLLY_STORAGE_CONSTEXPR decltype(groupVarintLengths) groupVarintLengths =
     make_array_with<256>(group_varint_table_length_make_item{});
 
 } // namespace detail

folly/Portability.h

@@ -366,6 +366,29 @@ constexpr auto kMscVer = 0;
 #define FOLLY_CPP14_CONSTEXPR inline
 #endif
 
+//  MSVC does not permit:
+//
+//    extern int const num;
+//    constexpr int const num = 3;
+//
+//  Instead:
+//
+//    extern int const num;
+//    FOLLY_STORAGE_CONSTEXPR int const num = 3;
+//
+//  True for MSVC 2015 and MSVC 2017.
+#if _MSC_VER
+#define FOLLY_STORAGE_CONSTEXPR
+#define FOLLY_STORAGE_CPP14_CONSTEXPR
+#else
+#define FOLLY_STORAGE_CONSTEXPR constexpr
+#if FOLLY_USE_CPP14_CONSTEXPR
+#define FOLLY_STORAGE_CPP14_CONSTEXPR constexpr
+#else
+#define FOLLY_STORAGE_CPP14_CONSTEXPR
+#endif
+#endif
+
 #if __cpp_coroutines >= 201703L && FOLLY_HAS_INCLUDE(<experimental/coroutine>)
 #define FOLLY_HAS_COROUTINES 1
 #elif _MSC_VER && _RESUMABLE_FUNCTIONS_SUPPORTED

folly/String.cpp

@@ -25,6 +25,7 @@
 
 #include <glog/logging.h>
 
+#include <folly/Portability.h>
 #include <folly/ScopeGuard.h>
 #include <folly/container/Array.h>
 
@@ -111,13 +112,13 @@ struct string_table_uri_escape_make_item {
   }
 };
 
-constexpr decltype(cEscapeTable) cEscapeTable =
+FOLLY_STORAGE_CONSTEXPR decltype(cEscapeTable) cEscapeTable =
     make_array_with<256>(string_table_c_escape_make_item{});
-constexpr decltype(cUnescapeTable) cUnescapeTable =
+FOLLY_STORAGE_CONSTEXPR decltype(cUnescapeTable) cUnescapeTable =
     make_array_with<256>(string_table_c_unescape_make_item{});
-constexpr decltype(hexTable) hexTable =
+FOLLY_STORAGE_CONSTEXPR decltype(hexTable) hexTable =
     make_array_with<256>(string_table_hex_make_item{});
-constexpr decltype(uriEscapeTable) uriEscapeTable =
+FOLLY_STORAGE_CONSTEXPR decltype(uriEscapeTable) uriEscapeTable =
     make_array_with<256>(string_table_uri_escape_make_item{});
 
 } // namespace detail