[ELF] - Implement linker script OVERLAYs.
This is PR36768.
Linker script OVERLAYs are described in 4.6.9. Overlay Description of the spec:
They are used to allow output sections which have different LMAs but the same VAs
and used for embedded programming.
Currently, LLD restricts overlapping of sections and that seems to be the most desired
behaviour for defaults. My thoughts about possible approaches for PR36768 are on the bug page,
this patch implements OVERLAY keyword and allows VAs overlapping for sections that within the overlay.
Differential revision: https://reviews.llvm.org/D44780