CodeGen: Allow aliases to be overridden by variables

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224827 91177308-0d34-0410-b5e6-96231b3b80d8
CodeGen: Error on redefinitions instead of asserting

It's possible to have a prior definition of a symbol in module asm.
Raise an error instead of crashing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224828 91177308-0d34-0410-b5e6-96231b3b80d8