[lld] [lld][ELF] Fix crash when relaxation pass encounters synthetic sections (PR #184758)

[Fangrui Song via llvm-commits]

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+# REQUIRES: riscv
+# RUN: rm -rf %t && split-file %s %t
+# RUN: llvm-mc --filetype=obj -triple=riscv64 %t/a.s -o %t/a.o
+# RUN: ld.lld -T %t/a.ld %t/a.o -o /dev/null
+
+## This test ensures that the relaxation pass does not crash when it encounters
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MaskRay wrote:

This first part can be deleted: `Do not crash when we encounter a synthetic section`...

Consider also testing a synthetic section without relocations, e.g. .dynsym  if you use `-pie`

https://github.com/llvm/llvm-project/pull/184758