[LLD][NFC] Remove getOffsetInFile() workaround.

Summary:
LLD has workaround for the times when SectionIndex was not passed properly:

LT->getFileLineInfoForAddress(
      S->getOffsetInFile() + Offset, nullptr,
      DILineInfoSpecifier::FileLineInfoKind::AbsoluteFilePath, Info));

S->getOffsetInFile() was added to differentiate offsets between
various sections. Now SectionIndex is properly specified.
Thus it is not necessary to use getOffsetInFile() workaround.
See https://reviews.llvm.org/D58194, https://reviews.llvm.org/D58357.

This patch removes getOffsetInFile() workaround.

Reviewers: ruiu, grimar, MaskRay, espindola

Reviewed By: grimar, MaskRay

Subscribers: emaste, arichardson, llvm-commits

Tags: #llvm, #lld

Differential Revision: https://reviews.llvm.org/D75636