[llvm-branch-commits] [NFCI] Avoid adding duplicated SpecialCaseList::Sections. (PR #140821)

[Qinkun Bao via llvm-branch-commits]

https://github.com/qinkunbao created https://github.com/llvm/llvm-project/pull/140821

https://github.com/llvm/llvm-project/pull/140127 converts SpecialCaseList::Sections
from StringMap to vector. However, the previous StringMap ensures that only a new
section is created when the SectionStr is different. We should keep the same behavior.