[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.

Depends on D146778

Reviewed By: MaskRay

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