[libcxx] msvc cmake

[G M]

Sorry I meant /W3 is the default. /W4 would be nice in some ways,
but /W4 warns about while (true) which isn't helpful.
I could replace the while (true) with for (;;) in libcxx.
With that change /W4 seems desirable but I'm not exactly sure what script
would add /W4 that still leaves things so they can be overridden on the
command line that isn't error prone.

I'm sure it can't be that hard if there is any interest. Otherwise I'll
leave things as they are with the prior patch which is /W3.
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