[libc++] Conflict between "_UI" template parameter and Windows tchar.h macro

At present `algorithm' and `random' can not be included after `tchar.h' [1]
because it contains

     #define _UI unsigned int

and the former contain templates like

     template <class _UI, _UI _Xp> struct __log2

that fail to compile with

     algorithm:2807: error C2332: 'class': missing tag name
     algorithm:2807: error C2628: '<unnamed-tag>' followed by 'unsigned' is illegal (did you forget a ';'?)
     algorithm:2807: error C2628: '<unnamed-tag>' followed by 'int' is illegal (did you forget a ';'?)
     algorithm:2807: error C2993: '': illegal type for non-type template parameter '<unnamed-tag>'

This affects compilation of open source libraries, in particular this
BDB file [2] fails to compile because it includes tchar.h [3] before C++
STL. What is the best way to resolve this? May libc++ rename or
undefine `_UI'?

[1] This one is from the old vc6 crt library, but Visual Studio 2017
    installs crt with the same macros:
[2] https://github.com/berkeleydb/libdb/blob/master/lang/cxx/cxx_seq.cpp
[3] https://github.com/berkeleydb/libdb/blob/master/src/dbinc/win_db.h#L33

That is annoying, but it seems like something that we probably should work around in libc++.

Very annoying indeed.

Fixed in r304348. I also asked some Windows STL devs is there are other macros we need to look out for.