[clangd] Implement iterator cost

[clangd] Implement iterator cost

This patch introduces iterator cost concept to improve the performance
of Dex query iterators (mainly, AND iterator). Benchmarks show that the
queries become ~10% faster.

Before

```
-------------------------------------------------------
Benchmark                Time           CPU Iteration
-------------------------------------------------------
DexAdHocQueries    5883074 ns    5883018 ns        117
DexRealQ         959904457 ns  959898507 ns          1
```

After

```
-------------------------------------------------------
Benchmark                Time           CPU Iteration
-------------------------------------------------------
DexAdHocQueries    5238403 ns    5238361 ns        130
DexRealQ         873275207 ns  873269453 ns          1
```

Reviewed by: sammccall

Differential Revision: https://reviews.llvm.org/D51310

llvm-svn: 341057