Fix hierarchical document symbol

1. Fixed a bug on building document symbol tree: As sym2ds was updated in
place, nested funcs/types may be moved into children of another
lsDocumentSymbol before itself got processed.

2. Namespaces only have declarations, in the old implementation it wasn't included in the result, making the result less hierarchical. This
commit fixes this by including the declarations of a symbol if no
definitions found.

Simplify semantic highlighting; improve hover of auto &&

Fix typo error (MaskRay#49)

- The word strengthened is spelled mistakenly.
- Unpaired symbol:')' seems to be missing.

Uniquify textDocument/references

There can be duplicates with template instantiation.

filesystem.cc: deduplicate with UniqueID

Congratulations to Tea Deliverers