Correct exception wrapper MSVC conditional

Summary:
The layout of an exception_ptr is determined by the ABI, not the OS. MinGW (and
Cygwin, depending on the build settings) define `_WIN32` and would therefore
have `kIsWindows` be true, but they use the Itanium ABI instead of Microsoft's,
so we shouldn't take the MSVC codepath for them.

Unfortunately, there isn't a reliable preprocessor method to determine the ABI
being used. `_MSC_VER` can be defined even when using the Itanium ABI (e.g.
when using windows-itanium triples with clang), and `_MSC_VER` can be undefined
even when using the Microsoft ABI (e.g. when building with `-fno-ms-extensions`
with clang).

To address this, introduce `kMicrosoftAbiVer`, which defaults to `_MSC_VER`,
but can be overridden manually if necessary. This should do the right thing by
default most of the time, while providing an escape hatch for when `_MSC_VER`
would be incorrect to use directly.

Reviewed By: yfeldblum

Differential Revision: D7377853

fbshipit-source-id: dd868370e9a496b98dd26dec77c4a75c49147c3c

folly/CPortability.h

@@ -163,3 +163,10 @@
 #else
 #define FOLLY_ATTR_WEAK
 #endif
+
+// Microsoft ABI version (can be overridden manually if necessary)
+#ifndef FOLLY_MICROSOFT_ABI_VER
+#ifdef _MSC_VER
+#define FOLLY_MICROSOFT_ABI_VER _MSC_VER
+#endif
+#endif

folly/ExceptionWrapper-inl.h

@@ -93,14 +93,14 @@ inline std::exception const* exception_wrapper::as_exception_or_null_(
 }
 
 static_assert(
-    !kIsWindows || (kMscVer >= 1900 && kMscVer <= 2000),
+    !kMicrosoftAbiVer || (kMicrosoftAbiVer >= 1900 && kMicrosoftAbiVer <= 2000),
     "exception_wrapper is untested and possibly broken on your version of "
     "MSVC");
 
 inline std::uintptr_t exception_wrapper::ExceptionPtr::as_int_(
     std::exception_ptr const& ptr,
     std::exception const& e) {
-  if (!kIsWindows) {
+  if (!kMicrosoftAbiVer) {
     return reinterpret_cast<std::uintptr_t>(&e);
   } else {
     // On Windows, as of MSVC2017, all thrown exceptions are copied to the stack

folly/Portability.h

@@ -357,6 +357,12 @@ constexpr auto kMscVer = _MSC_VER;
 constexpr auto kMscVer = 0;
 #endif
 
+#if FOLLY_MICROSOFT_ABI_VER
+constexpr auto kMicrosoftAbiVer = FOLLY_MICROSOFT_ABI_VER;
+#else
+constexpr auto kMicrosoftAbiVer = 0;
+#endif
+
 // TODO: Remove when removing support for gcc4.9
 #if __GLIBCXX__ && __GLIBCXX__ == 20150123
 constexpr auto kIsGlib49 = true;