[msvc] Strip /W[0-4] before appending /W4

Certain versions of CMake specify /W3 as part of CMAKE_CXX_FLAGS
by default, before you do anything.  Appending /W4 to the end of
this and using the Ninja generator results in

cl : Command line warning D9025 : overriding '/W3' with '/W4'.

It is not possible to suppress this since it is a command line
warning and not a compiler warning, so we must fix the command
line to contain only one value for /Wn.

llvm-svn: 234907