[clangd] Group filename calculations in SymbolCollector, and cache mroe.

Also give CanonicalIncludes a less powerful interface (canonicalizes
symbols vs headers separately) so we can cache its results better.

Prior to this:
 - path->uri conversions were not consistently cached, this is
   particularly cheap when we start from a FileEntry* (which we often can)
 - only a small fraction of header-to-include calculation was cached

This is a significant speedup at least for dynamic indexing of preambles.
On my machine, opening XRefs.cpp:

```
PreambleCallback 1.208 -> 1.019 (-15.7%)
BuildPreamble    5.538 -> 5.214 (-5.8%)
```

Differential Revision: https://reviews.llvm.org/D98371