New system for choosing CRT type on MSVC.

Is setted on a per build type basis. This is useful for generators
that comprise several build types, i.e. VC++ solutions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110296 91177308-0d34-0410-b5e6-96231b3b80d8