[ELF] mergeSections: remove non-alive MergeInputSection

Summary: This makes it conform to what the comment says. Otherwise when getErrPlace() is called afterwards, cast<InputSection>(D) will cause incompatible cast as MergeInputSection is not a subclass of InputSection.

Reviewers: ruiu, grimar, espindola, pcc

Reviewed By: grimar

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D50742

llvm-svn: 339904