[include-fixer] Remove line number from Symbol identity

Summary:
Remove line number from Symbol identity.

For our purposes (include-fixer and clangd autocomplete), function overloads
within the same header should mostly be treated as a single combined symbol.

We may want to track individual occurrences (line number, full type info)
and aggregate this during mapreduce, but that's not done here.

Reviewers: hokein, bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D30685

llvm-svn: 297371