[llvm-exegesis] InstructionBenchmarkClustering::dbScan(): use...

[llvm-exegesis] InstructionBenchmarkClustering::dbScan(): use llvm::SetVector<> instead of ILLEGAL std::unordered_set<>

Summary:
Test data: 500kLOC of benchmark.yaml, 23Mb. (that is a subset of the actual uops benchmark i was trying to analyze!)
Old time:
```
$ time ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=100000 -benchmarks-file=/tmp/benchmarks.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html &> /dev/null

real    0m24.884s
user    0m24.099s
sys     0m0.785s
```
New time:
```
$ time ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=100000 -benchmarks-file=/tmp/benchmarks.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html &> /dev/null

real    0m10.469s
user    0m9.797s
sys     0m0.672s
```
So -60%. And that isn't the good bit yet.

Old:
* calls to allocation functions: 106560180  (yes, 107 *million* allocations.)
* bytes allocated in total (ignoring deallocations): 12.17 GB

New:
* calls to allocation functions: 3347676  (-96.86%)  (just 3 mil)
* bytes allocated in total (ignoring deallocations): 10.52 GB (~2GB less)

---

Two points i want to raise:
* `std::unordered_set<>` should not have been used there in the first place.
  It is banned by the https://llvm.org/docs/ProgrammersManual.html#other-set-like-container-options
* There is no tests, so i'm not fully sure this is correct.
  Since it was unordered set, i guess there are zero restrictions on the order, and anything will be ok?
* I tried other containers suggested in https://llvm.org/docs/ProgrammersManual.html#set-like-containers-std-set-smallset-setvector-etc,
  this `llvm::SetVector<>` seems to be best here.

Reviewers: courbet, MaskRay, RKSimon, gchatelet, john.brawn

Reviewed By: courbet

Subscribers: kristina, bobsayshilol, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D54381

llvm-svn: 347197