Simplify exception handling

Simplify exception handling

Now that cleanups have been removed, TRY/CATCH can't be SJLJ-based any
more.  This patch simplifies the exception handling code, by removing
the non-working variants.

Note that the "pure" C++ exception handling code is removed as well; I
think the route forward must be to change exceptions to be
self-destructing, so that try_scope_depth can simply be removed.

Some longjmp-based code remains, as it is needed to throw an exception
through readline.

gdb/ChangeLog
2019-04-08  Tom Tromey  <tom@tromey.com>

	* common/common-exceptions.h (GDB_XCPT_SJMP, GDB_XCPT_TRY)
	(GDB_XCPT_RAW_TRY, GDB_XCPT): Remove.
	(TRY, CATCH, END_CATCH): Remove some definitions.
	* common/common-exceptions.c: Don't use GDB_XCPT.
	(catcher_list_size): Remove.
	(throw_exception, throw_it): Simplify.