[ELF] Better resemble GNU ld when placing orphan sections into memory regions

An orphan section should be placed in the same memory region as its
anchor section if the latter specifies the memory region explicitly.
If there is no explicit assignment for the anchor section in the linker
script, its memory region is selected by matching attributes, and the
same should be done for the orphan section.

Before the patch, some scripts that were handled smoothly in GNU ld
caused an "error: no memory region specified for section" in lld.

Differential Revision: https://reviews.llvm.org/D112925