Reland "[gold] Add preliminary FatLTO support to the Gold plugin""

This changes the definition if isSectionBitcode to only be valid for the
.llvm.lto section, since this API is only called from LTO, and the
.llvmbc section was not intended to be used for LTO. This allows the
gold plugin to keep its existing behavior without introducing any
significant changes.

Reviewed By: MaskRay, nikic

Differential Revision: https://reviews.llvm.org/D152973