[libcxxabi] Use the right calling convention for exception destructors on i386 Windows

[mstorsjo]

Summary

On Windows on i386, C++ member functions use a different calling
convention (__thiscall) than the default one for regular functions
(__cdecl). (On Windows on architectures other than i386, both calling
convention attributes are no-ops.)

This matches how libstdc++ declares these types.

This fixes the std/thread/futures/futures.{shared,unique}_future/dtor.pass.cpp
tests on i386 mingw.

libcxxabi/include/__cxxabi_config.h

 #if defined(_LIBCXXABI_COMPILER_CLANG)
 #  if !__has_feature(cxx_exceptions)
 #    define _LIBCXXABI_NO_EXCEPTIONS
 #  endif
 #elif defined(_LIBCXXABI_COMPILER_GCC) && !defined(__EXCEPTIONS)
 #  define _LIBCXXABI_NO_EXCEPTIONS
 #endif
 
+#if defined(_WIN32)
+#define _LIBCXXABI_DTOR_FUNC __thiscall
+#else
+#define _LIBCXXABI_DTOR_FUNC
+#endif
+
 #endif // ____CXXABI_CONFIG_H

libcxxabi/include/cxxabi.h

 extern _LIBCXXABI_FUNC_VIS void *
 __cxa_allocate_exception(size_t thrown_size) throw();
 extern _LIBCXXABI_FUNC_VIS void
 __cxa_free_exception(void *thrown_exception) throw();
 
 // 2.4.3 Throwing the Exception Object
 extern _LIBCXXABI_FUNC_VIS _LIBCXXABI_NORETURN void
 __cxa_throw(void *thrown_exception, std::type_info *tinfo,
-            void (*dest)(void *));
+            void (_LIBCXXABI_DTOR_FUNC *dest)(void *));
 
 // 2.5.3 Exception Handlers
 extern _LIBCXXABI_FUNC_VIS void *
 __cxa_get_exception_ptr(void *exceptionObject) throw();
 extern _LIBCXXABI_FUNC_VIS void *
 __cxa_begin_catch(void *exceptionObject) throw();
 extern _LIBCXXABI_FUNC_VIS void __cxa_end_catch();
 #if defined(_LIBCXXABI_ARM_EHABI)

libcxxabi/src/cxa_exception.h

#if defined(__LP64__) || defined(_WIN64) || defined(_LIBCXXABI_ARM_EHABI)
     // For binary compatibility it is at the start of this
     // struct which is prepended to the object thrown in
     // __cxa_allocate_exception.
     size_t referenceCount;
 #endif
 
     //  Manage the exception object itself.
     std::type_info *exceptionType;
-    void (*exceptionDestructor)(void *);
+    void (_LIBCXXABI_DTOR_FUNC *exceptionDestructor)(void *);
     std::unexpected_handler unexpectedHandler;
     std::terminate_handler  terminateHandler;
 
     __cxa_exception *nextException;
 
     int handlerCount;
 
 #if defined(_LIBCXXABI_ARM_EHABI)

 // primaryException instead of referenceCount.
 struct _LIBCXXABI_HIDDEN __cxa_dependent_exception {
 #if defined(__LP64__) || defined(_WIN64) || defined(_LIBCXXABI_ARM_EHABI)
     void* reserve; // padding.
     void* primaryException;
 #endif
 
     std::type_info *exceptionType;
-    void (*exceptionDestructor)(void *);
+    void (_LIBCXXABI_DTOR_FUNC *exceptionDestructor)(void *);
     std::unexpected_handler unexpectedHandler;
     std::terminate_handler terminateHandler;
 
     __cxa_exception *nextException;
 
     int handlerCount;
 
 #if defined(_LIBCXXABI_ARM_EHABI)

libcxxabi/src/cxa_exception.cpp

 pointer to the thrown exception, which __cxa_throw itself received as an argument.
 __Unwind_RaiseException begins the process of stack unwinding, described
 in Section 2.5. In special cases, such as an inability to find a
 handler, _Unwind_RaiseException may return. In that case, __cxa_throw
 will call terminate, assuming that there was no handler for the
 exception.
 */
 void
-__cxa_throw(void *thrown_object, std::type_info *tinfo, void (*dest)(void *)) {
+__cxa_throw(void *thrown_object, std::type_info *tinfo, void (_LIBCXXABI_DTOR_FUNC *dest)(void *)) {
     __cxa_eh_globals *globals = __cxa_get_globals();
     __cxa_exception* exception_header = cxa_exception_from_thrown_object(thrown_object);
 
     exception_header->unexpectedHandler = std::get_unexpected();
     exception_header->terminateHandler  = std::get_terminate();
     exception_header->exceptionType = tinfo;
     exception_header->exceptionDestructor = dest;
     setOurExceptionClass(&exception_header->unwindHeader);