gdb: return optional from last_symtab_line, and use this more

I noticed that last_symtab_line is defined as returning an int, but if
the file associated with the symtab cannot be read then the function
returns false!

This commit updates last_symtab_line to return std::optional<int> and
replaces 'return false' with 'return {}'.

I then realised that last_symtab_line isn't actually used very often,
but we do use source_cache::get_line_charpos to perform the same job.
So I went through all uses of ::get_line_charpos and replaced them
with last_symtab_line where appropriate.  I think this makes it
clearer what we're actually trying to do.

There should be no user visible changes after this commit.

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

0 files changed, 0 insertions, 0 deletions