[libc++] Implement exception_ptr on Windows

Summary:
This patch implements exception_ptr on Windows using the `__ExceptionPtrFoo` functions provided by MSVC.

The `__ExceptionPtrFoo` functions are defined inside the C++ standard library, `msvcprt`, which is unfortunate because it requires libc++ to link to the MSVC STL. However this doesn't seem to cause any immediate problems. However to be safe I kept all usages within the libc++ dylib so that user programs wouldn't have to link to MSVCPRT as well.

Note there are still 2 outstanding exception_ptr/nested_exception test failures.

* `current_exception.pass.cpp` needs to be rewritten for the Windows exception_ptr semantics which copy the exception every time.
* `rethrow_if_nested.pass.cpp` need investigation. It hits a stack overflow, likely from recursion.

This patch also gets most of the `<future>` tests passing as well.

Reviewers: mclow.lists, compnerd, bcraig, rmaprath, majnemer, BillyONeal, STL_MSFT

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D32927

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302393 91177308-0d34-0410-b5e6-96231b3b80d8

src/support/runtime/exception_pointer_msvc.ipp

@@ -0,0 +1,94 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdio.h>
+#include <stdlib.h>
+
+_CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL __ExceptionPtrCreate(_Out_ void*);
+_CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL __ExceptionPtrDestroy(_Inout_ void*);
+_CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL __ExceptionPtrCopy(_Out_ void*,
+                                                              _In_ const void*);
+_CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL
+__ExceptionPtrAssign(_Inout_ void*, _In_ const void*);
+_CRTIMP2_PURE bool __CLRCALL_PURE_OR_CDECL
+__ExceptionPtrCompare(_In_ const void*, _In_ const void*);
+_CRTIMP2_PURE bool __CLRCALL_PURE_OR_CDECL
+__ExceptionPtrToBool(_In_ const void*);
+_CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL __ExceptionPtrSwap(_Inout_ void*,
+                                                              _Inout_ void*);
+_CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL
+__ExceptionPtrCurrentException(_Out_ void*);
+[[noreturn]] _CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL
+__ExceptionPtrRethrow(_In_ const void*);
+_CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL
+__ExceptionPtrCopyException(_Inout_ void*, _In_ const void*, _In_ const void*);
+
+namespace std {
+
+exception_ptr::exception_ptr() _NOEXCEPT { __ExceptionPtrCreate(this); }
+exception_ptr::exception_ptr(nullptr_t) _NOEXCEPT { __ExceptionPtrCreate(this); }
+
+exception_ptr::exception_ptr(const exception_ptr& __other) _NOEXCEPT {
+  __ExceptionPtrCopy(this, &__other);
+}
+exception_ptr& exception_ptr::operator=(const exception_ptr& __other) _NOEXCEPT {
+  __ExceptionPtrAssign(this, &__other);
+  return *this;
+}
+
+exception_ptr& exception_ptr::operator=(nullptr_t) _NOEXCEPT {
+  exception_ptr dummy;
+  __ExceptionPtrAssign(this, &dummy);
+  return *this;
+}
+
+exception_ptr::~exception_ptr() _NOEXCEPT { __ExceptionPtrDestroy(this); }
+
+exception_ptr::operator bool() const _NOEXCEPT {
+  return __ExceptionPtrToBool(this);
+}
+
+bool operator==(const exception_ptr& __x, const exception_ptr& __y) _NOEXCEPT {
+  return __ExceptionPtrCompare(&__x, &__y);
+}
+
+
+void swap(exception_ptr& lhs, exception_ptr& rhs) _NOEXCEPT {
+  __ExceptionPtrSwap(&rhs, &lhs);
+}
+
+exception_ptr __copy_exception_ptr(void* __except, const void* __ptr) {
+  exception_ptr __ret = nullptr;
+  if (__ptr)
+    __ExceptionPtrCopyException(&__ret, __except, __ptr);
+  return __ret;
+}
+
+exception_ptr current_exception() _NOEXCEPT {
+  exception_ptr __ret;
+  __ExceptionPtrCurrentException(&__ret);
+  return __ret;
+}
+
+_LIBCPP_NORETURN
+void rethrow_exception(exception_ptr p) { __ExceptionPtrRethrow(&p); }
+
+nested_exception::nested_exception() _NOEXCEPT : __ptr_(current_exception()) {}
+
+nested_exception::~nested_exception() _NOEXCEPT {}
+
+_LIBCPP_NORETURN
+void nested_exception::rethrow_nested() const {
+  if (__ptr_ == nullptr)
+    terminate();
+  rethrow_exception(__ptr_);
+}
+
+} // namespace std