Boost failed when build with permissive- + MSVC on windows.

Boost failed with error C3646, C4430, C3861 when build with permissive- on Windows, I use latest source on master branch. Could you please help take a look at this? Noted that this issue only found when compiles with unreleased vctoolset, that next release of MSVC will have this behavior.

**You can repro this issue as the steps below:**

1. git clone -c core.autocrlf=true --recursive https://github.com/boostorg/boost.git D:\Boost\src
2. Open a VS 2015 x86 command prompt and browse to D:\Boost\src
3. .\bootstrap
4. .\b2 headers variant=release --build-dir=..\out\Release --address-model=32
5. .\b2 variant=release --build-dir=..\out\Release --address-model=32

**Error info:**
.\boost/range/concepts.hpp(257): error C3646: 'n': unknown override specifier
.\boost/range/concepts.hpp(264): note: see reference to class template instantiation 'boost::range_detail::RandomAccessIteratorConcept' being compiled
.\boost/range/concepts.hpp(257): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
.\boost/range/concepts.hpp(247): error C3861: 'n': identifier not found
.\boost/range/concepts.hpp(248): error C3861: 'n': identifier not found
.\boost/range/concepts.hpp(249): error C3861: 'n': identifier not found
.\boost/range/concepts.hpp(250): error C3861: 'n': identifier not found
.\boost/range/concepts.hpp(251): error C3861: 'n': identifier not found
.\boost/range/concepts.hpp(252): error C3861: 'n': identifier not found