Avoid undefined behavior in Guile exception handling

Avoid undefined behavior in Guile exception handling

The Guile code will longjmp (via scm_throw) when an object requiring
destruction is on the stack.  This is undefined behavior.

This changes this code to run any destructors in inner scopes, and to
pass a POD to gdbscm_throw_gdb_exception.

gdb/ChangeLog
2019-04-25  Tom Tromey  <tromey@adacore.com>

	* guile/scm-exception.c (gdbscm_scm_from_gdb_exception)
	(gdbscm_throw_gdb_exception): Take a gdbscm_gdb_exception.
	* guile/scm-block.c, guile/scm-breakpoint.c, guile/scm-cmd.c,
	guile/scm-disasm.c, guile/scm-frame.c, guile/scm-lazy-string.c,
	guile/scm-math.c, guile/scm-param.c, guile/scm-ports.c,
	guile/scm-symbol.c, guile/scm-symtab.c, guile/scm-type.c,
	guile/scm-value.c: Use unpack.
	* guile/guile-internal.h (gdbscm_scm_from_gdb_exception): Take a
	gdbscm_gdb_exception.
	(gdbscm_throw_gdb_exception): Likewise.
	(struct gdbscm_gdb_exception): New.
	(unpack): New function.
	(gdbscm_wrap): Use unpack.