Option parsing: allow aliases in groups

Option aliases in option groups were previously disallowed by an assert.
As far as I can tell, there was no technical reason for this, and I would
like to be able to put cl.exe compatible options in their own group for Clang,
so let's change the assert.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186838 91177308-0d34-0410-b5e6-96231b3b80d8