Make exception throwing a bit more efficient

Make exception throwing a bit more efficient

This makes exception throwing a bit more efficient, by removing some
copies.

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

	* common/common-exceptions.c (throw_exception): Rename from
	throw_exception_cxx.  Remove old copy.  Make argument const.
	(throw_it): Create and throw exception objects directly.
	* common/common-exceptions.h (throw_exception): Make argument
	const.
	(struct gdb_exception_error): Add constructor.
	(struct gdb_exception_quit): Add constructor.