Currently, when breakpoint source tracking is enabled (see 'set
breakpoint source-tracking enabled on'), GDB will not source track
multi-location breakpoints.

As a first take this was fine, but I think that we can support source
tracking for some multi-location breakpoints.

Source tracking only applies to breakpoints set based on a file and
line number, so the rest of this discussion is only concerned with
this case.  This includes things like 'break file:line' and also
'break line'.  In the latter case GDB uses the current source file to
provide the filename, so the two cases are really equivalent.

If we consider 'break file:line' then there are two ways that this can
resolve to multiple locations.

First, if 'file:line' lands in an inline function, and the inline
function is inlined multiple times (or has an out-of-line
instantiation) then there can be multiple locations.  However, there
is only a single source line for all of these breakpoint locations,
the original line in the single original file.