Handle section vs global name conflict.

This is a fix for PR26941.

When there is both a section and a global definition with the same
name, the global wins.

Section symbols are not added to the symbol table; section references
are left undefined and fixed up in the object writer unless they've
been satisfied by some other definition.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264649 91177308-0d34-0410-b5e6-96231b3b80d8