Only compute realpath when basenames_may_differ is set

Only compute realpath when basenames_may_differ is set

A user noted that, when sources are symlinked, gdb annotations will
print the real path, rather than the name of the symlink.

It seems to me that it is better to print the name of the file that
was actually used in the build, unless there is some reason not to.

This patch implements this, with the caveat that it will not work when
basenames-may-differ is enabled.  The way this mode is currently
implemented, returning the symbolic (not real) path is not possible.

While I think it would be good to redo the source file name cache and
perhaps integrate it with class source_cache, I haven't done so here.

Regression tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	* source.c (find_and_open_source): Respect basenames_may_differ.

gdb/testsuite/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	* gdb.base/annotate-symlink.exp: New file.