[lld][MachO] Tail merge strings (#161262)

Add the flag `--tail-merge-strings` to enable tail merging of cstrings.
For example, if we have strings `mystring\0` and `ring\0`, we could
place `mystring\0` at address `0x1000` and `ring\0` at address `0x1004`
and have them share the same underlying data.

It turns out that many ObjC method names can be tail merged. For
example, `error:` and `doFoo:error:`. On a large iOS binary, we saw
nearly a 15% size improvement in the `__TEXT__objc_methname` section and
negligible impact on link time.
```
$ bloaty --domain=vm merged.o.stripped -- base.o.stripped
     VM SIZE
 --------------
   +95% +5.85Ki    [__TEXT]
  -2.4%  -239Ki    __TEXT,__cstring
 -14.5%  -710Ki    __TEXT,__objc_methname
  -1.0%  -944Ki    TOTAL
```

Tail merging for MachO was originally removed in
https://github.com/llvm/llvm-project/commit/7c269db779ff3950bac2e25ea78b14b4e2b8b247.
The previous implementation used `StringTableBuilder`, but that was
removed in
https://github.com/llvm/llvm-project/commit/4308f031cd0c679c539914608134b9c8046743b3
to ensure deduplicated strings are aligned correctly. This
implementation ensures that tail merged strings are also aligned
correctly.

Special thanks to nocchijiang for pointing this out in
https://github.com/llvm/llvm-project/pull/158720#issuecomment-3310416030.

Depends on https://github.com/llvm/llvm-project/pull/161253.

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