[cmake] Replace -Wall with /W4 in clang-cl options now that -Wall aliases -Weverything
[cmake] Replace -Wall with /W4 in clang-cl options now that -Wall aliases -Weverything

Instead, reuse the code-path for cl.exe that adds /W4 , which for clang-cl
aliases clang's "-Wall -Wextra" which matches what clang-cl's /Wall
previously aliased.

This should restore the verbosity of a Windows selfhost build back to
its previous levels.

Differential Revision: https://reviews.llvm.org/D40603

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@319330 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm