[Object] Add ELF section type SHT_LLVM_BITCODE for LLVM bitcode

clang -ffat-lto-objects can use this new ELF section type for the .llvm.lto
section for fat LTO support (D146776).

Original RFC: https://discourse.llvm.org/t/rfc-ffat-lto-objects-support/63977

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D153215