Change symbol::line to unsigned int

Change symbol::line to unsigned int

A user here at AdaCore noticed that, when debugging a certain program,
a stack frame reported line 34358, where it should have been line
99894.

After debugging a bit, I discovered:

(top) p (99894 & ~65536)
$60 = 34358

That line, symbol::line is too narrow.

This patch widens the member and changes all the uses that currently
use the narrower type.

Approved-By: Simon Marchi <simon.marchi@efficios.com>