Reland: [LLD] Implement --enable-non-contiguous-regions (#90007)

When enabled, input sections that would otherwise overflow a memory
region are instead spilled to the next matching output section.

This feature parallels the one in GNU LD, but there are some differences
from its documented behavior:

- /DISCARD/ only matches previously-unmatched sections (i.e., the flag
does not affect it).

- If a section fails to fit at any of its matches, the link fails
instead of discarding the section.

- The flag --enable-non-contiguous-regions-warnings is not implemented,
as it exists to warn about such occurrences.

The implementation places stubs at possible spill locations, and
replaces them with the original input section when effecting spills.
Spilling decisions occur after address assignment. Sections are spilled
in reverse order of assignment, with each spill naively decreasing the
size of the affected memory regions. This continues until the memory
regions are brought back under size. Spilling anything caus...