fix(demangle): support decltype(nullptr) and pack expansion

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (45d7978) 64.41% compared to head (f6ccf02) 64.42%.

Files	Patch %	Lines
src/demangle.cc	83.33%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1018 +/- ## ========================================== + Coverage 64.41% 64.42% +0.01% ========================================== Files 18 18 Lines 3229 3233 +4 Branches 1071 1072 +1 ========================================== + Hits 2080 2083 +3 Misses 727 727 - Partials 422 423 +1 ```

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