Add fuzzy SymbolIndex, where identifier needn't match exactly.
The purpose for this is global autocomplete in clangd. The query will be a
partial identifier up to the cursor, and the results will be suggestions.
It's in include-fixer because:
- it handles SymbolInfos, actually SymbolIndex is exactly the right interface
- it's a good harness for lit testing the fuzzy YAML index
- (Laziness: we can't unit test clangd until reorganizing with a tool/ dir)
Other questionable choices:
- FuzzySymbolIndex, which just refines the contract of SymbolIndex. This is an interface to allow extension to large monorepos (*cough*)
- an always-true safety check that Identifier == Name is removed from SymbolIndexManager, as it's not true for fuzzy matching
- exposing -db=fuzzyYaml from include-fixer is not a very useful feature, and a non-orthogonal ui (fuzziness vs data source). -db=fixed is similar though.