[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
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 15dc05c..6161838 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -121,6 +121,7 @@
   add_library_flags(ucrt${LIB_SUFFIX}) # Universal C runtime
   add_library_flags(vcruntime${LIB_SUFFIX}) # C++ runtime
   add_library_flags(msvcrt${LIB_SUFFIX}) # C runtime startup files
+  add_library_flags(msvcprt${LIB_SUFFIX}) # C++ standard library. Required for exception_ptr internals.
   # Required for standards-complaint wide character formatting functions
   # (e.g. `printfw`/`scanfw`)
   add_library_flags(iso_stdio_wide_specifiers)