Extend -Rmodule-build to also remark when module building finishes.

In cases of nested module builds, or when you care how long module builds take,
this information was not previously easily available / obvious.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219658 91177308-0d34-0410-b5e6-96231b3b80d8