[ELF] Drop const qualifier to fix -Wrange-loop-analysis. NFC

```
lld/ELF/Relocations.cpp:1622:56: warning: loop variable 'ts' of type 'const std::pair<ThunkSection *, uint32_t>' (aka 'const pair<lld::elf::ThunkSection *, unsigned int>') creates a copy from type 'const std::pair<ThunkSection *, uint32_t>' [-Wrange-loop-analysis]
        for (const std::pair<ThunkSection *, uint32_t> ts : isd->thunkSections)
```

Drop const qualifier to fix -Wrange-loop-analysis.
We can make -Wrange-loop-analysis warnings (DiagnoseForRangeConstVariableCopies) on `const A` more
permissive on more types (e.g. POD -> trivially copyable), unfortunately it will not make std::pair
good, because `constexpr pair& operator=(const pair& p);` is unfortunately user-defined.

Reviewed By: Mordante

Differential Revision: https://reviews.llvm.org/D72211