[Ada catchpoints] Fix "warning: failed to get exception name: No definition of \...

[Ada catchpoints] Fix "warning: failed to get exception name: No definition of \"e.full_name\" in current context"

Looking at testsuite results, I noticed this warning in an MI test:

 ~"\nCatchpoint "
 ~"2, "
 &"warning: failed to get exception name: No definition of \"e.full_name\" in current context.\n"
 ~"exception at 0x000000000040192d in foo () at /home/pedro/brno/pedro/gdb/mygit/src/gdb/testsuite/gdb.ada/mi_catch_ex/foo.adb:20\n"
 ~"20\t      raise Constraint_Error;  -- SPOT1\n"
 *stopped,reason="breakpoint-hit",disp="keep",bkptno="2",exception-name="CONSTRAINT_ERROR",frame={addr="0x000000000040192d",func="foo",args=[],file="/home/pedro/brno/pedro/gdb/mygit/src/gdb/testsuite/gdb.ada/mi_catch_ex/foo.adb",fullname="/home/pedro/brno/pedro/gdb/mygit/src/gdb/testsuite/gdb.ada/mi_catch_ex/foo.adb",line="20"},thread-id="1",stopped-threads="all",core="5"
 (gdb)
 PASS: gdb.ada/mi_catch_ex.exp: continue until CE caught by all-exceptions catchpoint

The problem is that:

  - MI prints the breakpoint hit twice: once on the MI stream;
    another time on the console stream.

  - After printing the Ada catchpoint hit, gdb selects a non-current
    frame, from within the catchpoint's print_it routine.

So the second time the breakpoint is printed, the selected frame is no
longer the current frame, and then evaluating e.full_name in
ada_exception_name_addr fails.

This commit fixes the problem and enhances the gdb.ada/mi_catch_ex.exp
test to make sure the catchpoint hit is printed correctly on the
console stream too.

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* ada-lang.c (ada_exception_name_addr_1): Add comment.
	(print_it_exception): Select the current frame.

gdb/testsuite/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* gdb.ada/mi_catch_ex.exp (continue_to_exception): New procedure.
	(top level): Use it instead of mi_execute_to.