gdb/testsuite: fix gdb.reverse/solib-*.exp tests when using clang

gdb/testsuite: fix gdb.reverse/solib-*.exp tests when using clang

The tests gdb.reverse/solib-precsave.exp and solib-reverse.exp have the
assumption that line tables will have an entry for the closing } in a
function. Not all compiles do this, one example being clang. To fix
this, this commit changes the function in shr2.c to have multiple lines,
and the test to accept either line as a correct step location.

To properly re-sync the inferiors, the function repeat_cmd_until had to
be slightly changed to work with empty "current locations", so that we
are able to step through multiple lines.

This also changes the annotations used to determine the breakpoint
locations in solib-reverse.c, adding a simple variable assignment right
before the return statement. This way GDB will not set a breakpoint in
the closing } line.

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