Switch Wmodule-build to a remark

On reflection, this is better despite the missing command-line handling
bits for remarks.  Making this a remark makes it much clearer that
this is purely informational and avoids the negative connotations of a
'warning'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208367 91177308-0d34-0410-b5e6-96231b3b80d8