G M: Attached is a patch for libcxx's cmake file.
 
I've changed it so we don't set highest level warnings (all) for MSVC when building projects using cmake and instead leave the default. That's /W4 on my machine and seems to be ok.
 
With all warnings on for msvc, we see literally thousands of warnings. 99.99% aren't relevant and just obscure the ones that are.
I think the user can still override things if they want something different from the command line when using cmake.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@192010 91177308-0d34-0410-b5e6-96231b3b80d8