SE-390 concluded with choosing the keyword discard rather than forget for
the statement that disables the deinit of a noncopyable type. This commit
adds parsing support for `discard self` and adds a deprecation warning for
`_forget self`.
rdar://108859077
the main things still left behind the experimental flag(s) are
- move-only classes (guarded by MoveOnlyClasses feature)
- noimplicitcopy
- the _borrow operator
Add new `-print-ast-decl` frontend option for only printing declarations,
to match existing behavior.
Some tests want to print the AST, but don't care about expressions.
The existing `-print-ast` option now prints function bodies and expressions.
Not all expressions are printed yet, but most common ones are.
