[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

llvm-svn: 319330