Produce errors when an assignment expression would use a common symbol.

Produce errors when an assignment expression would use a common symbol.

An assignment will produce a symbol with a given section and offset. There is
no way to represent something like "1 byte after a common symbol".

This matches the behavior of GNU as.

Part of PR22217.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226470 91177308-0d34-0410-b5e6-96231b3b80d8