[ELF] --emit-relocs: fix missing STT_SECTION when the first input section is synthetic

addSectionSymbols suppresses the STT_SECTION symbol if the first input section
is non-SHF_MERGE synthetic. This is incorrect when the first input section is synthetic
while a non-synthetic input section exists:

* `.bss : { *(COMMON) *(.bss) }`
  (abc388ed regressed the case because
  COMMON symbols precede .bss in the absence of a linker script)
* Place a synthetic section in another section: `.data : { *(.got) *(.data) }`

For `%t/a1` in the new test emit-relocs-synthetic.s, ld.lld produces incorrect
relocations with symbol index 0.
```
0000000000000000 <_start>:
       0: 8b 05 33 00 00 00             movl    51(%rip), %eax          # 0x39 <bss>
                0000000000000002:  R_X86_64_PC32        *ABS*+0xd
       6: 8b 05 1c 00 00 00             movl    28(%rip), %eax          # 0x28 <common>
                0000000000000008:  R_X86_64_PC32        common-0x4
       c: 8b 05 06 00 00 00             movl    6(%rip), %eax           # 0x18
                000000000000000e:  R_X86_64_GOTPCRELX   *ABS*+0x4
```

Fix the issue by checking every input section.

Reviewed By: ikudrin

Differential Revision: https://reviews.llvm.org/D122463