[clangd] Dex Trigrams: Improve query trigram generation

These are the trigrams for queries right now:

- "va" -> {Trigram("va")}
- "va_" -> {} (empty)

This is suboptimal since the resulting query will discard the query information
and return all symbols, some of which will be later be scored expensively
(fuzzy matching score). This is related to
https://github.com/clangd/clangd/issues/39 but does not fix it. Accidentally,
because of that incorrect behavior, when user types "tok::va" there are no
results (the issue is that `tok::kw___builtin_va_arg` does not have "va" token)
but when "tok::va_" is typed, expected result (`tok::kw___builtin_va_arg`)
shows up by accident. This is because the dex query transformer will only
lookup symbols within the `tok::` namespace. There won't be many, so the
returned results will contain symbol we need; this symbol will be filtered out
by the expensive checks and that will be displayed in the editor.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D113995

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