[libcxx] Initializer list support for MSVC

[K-ballo]

Summary

MSVC expects std::initializer_list to have a publicly accessible constructor taking the begin and end pointers.

[mclow.lists]

Do we need tests for this change?

[K-ballo]

In D5127#4, @mclow.lists wrote:

Do we need tests for this change?

The existing tests will do fine, the problem is that the test wont just run with libc++ as is. I had to make a number of changes and add workarounds to get it to work with MSVC, as well as hacked lit.cfg to just work. I'm not sure how to proceed with these changes.

[mclow.lists]

Do we need tests for this change?

include/initializer_list

83

Can we make this private?
We don't want anyone calling it inadvertently.

[mclow.lists]

Regarding my "private" comment: I checked with MS, and the answer is "no". Even thought this is only called by compiler-generated code, the compiler still pays attention to the access permissions.

This is, of course, fixable in their compiler, and I will enter a DR for this vs MSVC. But that doesn't help us today.

[K-ballo]

Waiting for VS2015RTM

include/initializer_list

public:
 
     typedef const _Ep* iterator;
     typedef const _Ep* const_iterator;
 
     _LIBCPP_ALWAYS_INLINE
     _LIBCPP_CONSTEXPR_AFTER_CXX11
     initializer_list() _NOEXCEPT : __begin_(nullptr), __size_(0) {}
 
+#ifdef _LIBCPP_MSVC
+    _LIBCPP_ALWAYS_INLINE
+    _LIBCPP_CONSTEXPR_AFTER_CXX11
+    initializer_list(const _Ep* __b, const _Ep* __e) _NOEXCEPT
+        : __begin_(__b),
+          __size_(__e - __b)
+        {}
+#endif  // _LIBCPP_MSVC
+
     _LIBCPP_ALWAYS_INLINE
     _LIBCPP_CONSTEXPR_AFTER_CXX11
     size_t    size()  const _NOEXCEPT {return __size_;}
     
     _LIBCPP_ALWAYS_INLINE
     _LIBCPP_CONSTEXPR_AFTER_CXX11
     const _Ep* begin() const _NOEXCEPT {return __begin_;}