[MC] Common symbols weren't being checked for redeclaration which allowed an assembly file to generate an assertion in setCommon(): !isCommon().  This change allows redeclaration as long as the size and alignment match exactly, otherwise report a fatal error.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239227 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed