[SpecialCaseList] Remove TrigramIndex

`TrigramIndex` was added back in https://reviews.llvm.org/D27188 as an optimization to make `SpecialCaseList::match()` faster. I've found that `TrigramIndex` actually makes the function slower and it has no functional use, so we can remove it.

I grabbed the list of queries passed to `SpecialCaseList::match()` on a random very large file (`AArch64ISelLowering.cpp`) and measured the runtime to call `match()` on all of them with [this line](https://github.com/llvm/llvm-project/blob/8e1f820bb4eadf5c0704818f6063e0db1006e32d/llvm/lib/Support/SpecialCaseList.cpp#L64) disabled and then enabled.

```
$ hyperfine --warmup 3 'GTEST_FILTER="SpecialCaseListTest.Large" USE_TRIGRAMS=1 build/unittests/Support/SupportTests' 'GTEST_FILTER="SpecialCaseListTest.Large" USE_TRIGRAMS=0 build/unittests/Support/SupportTests'
Benchmark 1: GTEST_FILTER="SpecialCaseListTest.Large" USE_TRIGRAMS=1 build/unittests/Support/SupportTests
  Time (mean ± σ):     575.9 ms ±  20.3 ms    [User: 573.1 ms, System: 2.7 ms]
  Range (min … max):   555.5 ms … 620.0 ms    10 runs

Benchmark 2: GTEST_FILTER="SpecialCaseListTest.Large" USE_TRIGRAMS=0 build/unittests/Support/SupportTests
  Time (mean ± σ):     283.4 ms ±   6.7 ms    [User: 280.3 ms, System: 3.0 ms]
  Range (min … max):   277.0 ms … 294.9 ms    10 runs

Summary
  'GTEST_FILTER="SpecialCaseListTest.Large" USE_TRIGRAMS=0 build/unittests/Support/SupportTests' ran
    2.03 ± 0.09 times faster than 'GTEST_FILTER="SpecialCaseListTest.Large" USE_TRIGRAMS=1 build/unittests/Support/SupportTests'
```

Using `perf` I found that most of the runtime in `TrigramIndex::isDefinitelyOut()` comes from a division operation that seems to come from `std::unordered_map`: https://github.com/llvm/llvm-project/blob/8e1f820bb4eadf5c0704818f6063e0db1006e32d/llvm/include/llvm/Support/TrigramIndex.h#L62

Removing `TrigramIndex` will make it easier to potentially switch to using `GlobPattern` instead of a full regex for `SpecialCaseList`. See discussion in https://reviews.llvm.org/D152762 for details.

Reviewed By: MaskRay, #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D153171