[lld/ELF] removeUnusedSyntheticSections can remove sections required later

For example as mentioned in https://github.com/llvm/llvm-project/pull/211911, `X86_64::relaxOnce` can add GOT entries for GOTPCRELX relocations that were relaxed to PC32 but actually would end up overflowing and we undo the relaxation. However, if there are no `.rela.dyn`/`.relr.dyn` entries, `removeUnusedSyntheticSections()` will remove them prior to `X86_64::relaxOnce` and we'll end up without the dynamic relocations.

```
$ cat repro.s
.text
.globl _start
_start:
movq foo@GOTPCREL(%rip), %rax

.section .data,"aw",@progbits
.globl foo
foo:
.byte 0

$ llvm-mc -filetype=obj -triple=x86_64 repro.s -o repro.o
$ ld.lld -pie --pack-dyn-relocs=relr --section-start=.text=0x10000 --section-start=.got=0x20000 --section-start=.data=0x100000000 repro.o -o repro
$ llvm-readobj -r repro

File: repro
Format: elf64-x86-64
Arch: x86_64
AddressSize: 64bit
LoadName:
Relocations [
]
```

#96496 has a similar issue with `.relr.auth.dyn` -> `.rela.dyn`.

Start with the repro.s assembly and run the listed llvm-mc, ld.lld, and llvm-readobj commands to reproduce the missing dynamic relocations. Trace removeUnusedSyntheticSections and X86_64::relaxOnce, including the related #96496 case; done means the required dynamic relocation sections remain available when relaxation is undone and the repro reports the expected relocations.

Written by the indexing model from the issue text.