fix(debuginfo): include more pdb symbols

[mstange]

Instead of checking symbol.function, check whether the symbol is contained
in an executable section. This matches what pdb-addr2line does, and is
closer to what dump_syms' handwritten PDB parsing does.

This change allows more symbols to be included. For example, it allows the
following symbols from basic-opt32.pdb to pass the check:

4f9d7 _rtinfpopse
4f9d9 _rtinfnpopse
4f9e9 _fFLN
4fa9e _rtinfpop
4faa0 _rtinfnpop
4fabd _ffexpm1

https://github.com/mozilla/dump_syms/blob/54f9e6240e34cf17fc7dc60ac4985772e2c20001/test_data/windows/basic-opt32.pdb

These symbols have symbol.function == false, so they were being filtered
out before.

The change also adds over 300 symbols for ntdll.pdb BD298DA990CD4BF9BE5CE4796D7924C61.

The handwritten PDB parsing in dump_syms currently uses this full check:
symbol.function || symbol.code || is_in_executable_section || is_in_executable_section_contribution

This seems like overkill; I've tested a few pdbs and haven't found any
for which this additional complexity would make a difference.

[codecov-commenter]

Codecov Report

Merging #641 (a5430ed) into master (a0360d3) will increase coverage by 0.04%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master     #641      +/-   ##
==========================================
+ Coverage   71.20%   71.25%   +0.04%     
==========================================
  Files          96       96              
  Lines       18408    18433      +25     
==========================================
+ Hits        13108    13134      +26     
+ Misses       5300     5299       -1