[clang] [FrontEnd] Use SetVector (NFC) (PR #107743)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve-vla-2stage` running on `linaro-g3-03` while building `clang` at step 11 "build stage 2".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/41/builds/1894

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 11 (build stage 2) failure: 'ninja' (failure)
...
[7829/8665] Building CXX object tools/flang/lib/Semantics/CMakeFiles/FortranSemantics.dir/check-nullify.cpp.o
[7830/8665] Building CXX object tools/flang/lib/Semantics/CMakeFiles/FortranSemantics.dir/check-purity.cpp.o
[7831/8665] Building CXX object tools/flang/lib/Semantics/CMakeFiles/FortranSemantics.dir/check-namelist.cpp.o
[7832/8665] Building CXX object tools/flang/lib/Semantics/CMakeFiles/FortranSemantics.dir/check-stop.cpp.o
[7833/8665] Building CXX object tools/flang/lib/Semantics/CMakeFiles/FortranSemantics.dir/check-if-stmt.cpp.o
[7834/8665] Building CXX object tools/flang/lib/Semantics/CMakeFiles/FortranSemantics.dir/program-tree.cpp.o
[7835/8665] Building CXX object tools/flang/lib/Semantics/CMakeFiles/FortranSemantics.dir/compute-offsets.cpp.o
[7836/8665] Building CXX object tools/flang/lib/Semantics/CMakeFiles/FortranSemantics.dir/check-io.cpp.o
[7837/8665] Building CXX object tools/flang/lib/Semantics/CMakeFiles/FortranSemantics.dir/check-select-rank.cpp.o
[7838/8665] Building CXX object tools/flang/lib/Semantics/CMakeFiles/FortranSemantics.dir/scope.cpp.o
command timed out: 1200 seconds without output running [b'ninja'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=3320.371399

```

</details>

https://github.com/llvm/llvm-project/pull/107743