[pseudo] Add benchmarks for pseudoparser.

Running on SemaDecl.cpp with the cxx.bnf grammar:

```
--------------------------------------------------------------
Benchmark                    Time             CPU   Iterations
--------------------------------------------------------------
runParseBNFGrammar      649389 ns       649365 ns         1013
runBuildLR            34591903 ns     34591380 ns           20
runPreprocessTokens   11418744 ns     11418703 ns           61 bytes_per_second=63.8971M/s
runGLRParse          282996863 ns    282988726 ns            2 bytes_per_second=2.57827M/s
runParseOverall      294969719 ns    294951870 ns            2 bytes_per_second=2.4737M/s
```

Differential Revision: https://reviews.llvm.org/D125226