Don't include an invalid symbol in the symbol table.

The symbol table itself has no relocations, so it is not possible to represent
things like

a = undefined + 1

With the patch we just omit these variables. That matches the behaviour of the
gnu assembler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207419 91177308-0d34-0410-b5e6-96231b3b80d8