[CMake] Adjust MSVC warnings.

Remove /Wall from LLVM_ENABLE_WARNINGS (it's useless) and promote 4239
to a level 1 warning.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158170 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed