[ELF] Reduce symbol partitions to shim binaries (#198718)
The experimental symbol partition feature can move arbitrary symbols and
input sections out of the main partition into named loadable partitions
that consumers extract with `llvm-objcopy --extract-partition`. It is
incompatible with many features, has seen no adoption beyond Chromium's
`build/extract_partition.py`. Chrome hasn't needed this feature for many
years.
The partitioning machinery (per-partition `MarkLive` runs, `moveToMain`,
cross-partition pull in `InputSection::replace`, per-symbol
dynsym/gnuhash/thunk-compat filters, `PartitionIndexSection`,
`copySectionsIntoPartitions`, etc.) does not pull its weight for that.
Remove the partitioning brain and keep a minimal shim shell.
`Symbol::partition` is deleted; every dynsym add goes to
`ctx.mainPart`. `ctx.partitions` wraps a `std::vector<Partition>`
whose iterator yields only the main partition; shim partitions live
in `storage[1..]` and are reached via `shims()`. The two paths that
actually emit shim binaries (`createPhdrs` and `pageAlign` in
Writer.cpp) name `shims()` directly.
Shim partitions carry no per-partition synthetic content; the
extracted shim is just an ELF header plus PT_PHDR/PT_LOAD.
`llvm-objcopy --extract-partition` keeps working but
`--dump-section .note.gnu.build-id` on a shim no longer finds
anything. Features that previously used `dlopen()` to lazy-load a
feature partition now live in main and are always loaded.
Aided by Claude Opus 4.7
