[gdb/symtab] Factor out new_symbol_file_line

I was looking at a patch [1] modifying the DW_AT_decl_file / DW_AT_call_file
handling of new_symbol, and noticed that:
- the code is fairly nested, and
- the patch adds another nesting layer.

Factor out function new_symbol_file_line out of new_symbol, handling both the
DW_AT_decl_file / DW_AT_call_file and DW_AT_call_line / DW_AT_decl_line
attributes.

Having factored out the code, simplify it using return, and modernize it using
bool and nullptr.

Approved-By: Tom Tromey <tom@tromey.com>

[1] https://sourceware.org/pipermail/gdb-patches/2026-March/226065.html