Fixes PR 25475: ensure exec-file-mismatch "ask" always asks in case of mismatch.

Fixes PR 25475: ensure exec-file-mismatch "ask" always asks in case of mismatch.

As explained in https://sourceware.org/bugzilla/show_bug.cgi?id=25475,
when the currently loaded file has no debug symbol,
symbol_file_add_with_addrs does not ask a confirmation to the user
before loading the new symbol file.  The behaviour is not consistent
when symbol_file_add_with_addrs is called due to exec-file-mismatch "ask"
setting.

The PR discusses several solutions/approaches.
The preferred approach (suggested by Joel) is to ensure that GDB always asks
a confirmation when it loads a new symbol file due to exec-file-mismatch,
using a new SYMFILE add-flag.

I tested this manually.  If OK, we can remove the bypass introduced by Tom
in 6b9374f1, in order to always answer to the 'load' question.

gdb/ChangeLog
2020-06-24  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* symfile-add-flags.h: New flag SYMFILE_ALWAYS_CONFIRM.
	* exec.c (validate_exec_file): If from_tty, set both
	SYMFILE_VERBOSE (== from_tty) and SYMFILE_ALWAYS_CONFIRM.
	* symfile.c (symbol_file_add_with_addrs): if always_confirm
	and from_tty, unconditionally ask a confirmation.