[PDB] Bypass generic deserialization code for publics sorting

[PDB] Bypass generic deserialization code for publics sorting

The number of public symbols is very large, and each deserialization
does a few heap allocations. The public symbols are serialized by the
linker, so we can assume they have the expected layout and use it
directly.

Saves O(#publics) temporary heap allocations and shrinks some data
structures.