[clang-cl] Alias /Wall to -Weverything

cl interprets this option to mean enable every supported warning, which
is what Clang's -Weverything flag does.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319116 91177308-0d34-0410-b5e6-96231b3b80d8