Summary:
There is no support in XCOFF for labels on common symbols. Therefore, an alias for a common symbol is not supported. Issue an error in the front end when an aliasee is a common symbol. Issue a similar error in the back end in case an IR specifies an alias for a common symbol.
Reviewed by: hubert.reinterpretcast, DiggerLin
Differential Revision: https://reviews.llvm.org/D158739
