[clangd] Index local classes, virtual and overriding methods.

Previously we did not record local class declarations. Now with features like
findImplementation and typeHierarchy, we have a need to index such local
classes to accurately report subclasses and implementations of methods.

Performance testing results:
- No changes in indexing timing.
- No significant change in memory usage.
- **1%** increase in #relations.
- **0.17%** increase in #refs.
- **0.22%** increase #symbols.

**New index stats**
Time to index: **4:13 min**
memory usage **543MB**
number of symbols: **521.5K**
number of refs: **8679K**
number of relations: **49K**

**Base Index stats**
Time to index: **4:15 min**
memory usage **542MB**
number of symbols: **520K**
number of refs: **8664K**
number of relations: **48.5K**

Fixes: https://github.com/clangd/clangd/issues/644

Differential Revision: https://reviews.llvm.org/D94785