This executable construct has a larger list of clauses than some of the
others, plus has some additional restrictions.  This patch implements
the AST node, plus the 'cannot be the body of a if, while, do, switch,
    or label' statement restriction.  Future patches will handle the
    rest of the restrictions, which are based on clauses.

A fairly simple one, only valid on the 'set' construct, this clause
takes an int expression.  Most of the work was already done as a part of
parsing, so this patch ends up being a lot of infrastructure.

The 'set' construct is another fairly simple one, it doesn't have an
associated statement and only a handful of allowed clauses. This patch
implements it and all the rules for it, allowing 3 of its for clauses.
The only exception is default_async, which will be implemented in a
future patch, because it isn't just being enabled, it needs a complete
new implementation.

This is a very simple sema implementation, and just required AST node
plus the existing diagnostics.  This patch adds tests and adds the AST
node required, plus enables it for 'init' and 'shutdown' (only!)

These two constructs are very simple and similar, and only support 3
different clauses, two of which are already implemented.  This patch
adds AST nodes for both constructs, and leaves the device_num clause
unimplemented, but enables the other two.

The arguments to this are the same as for the 'wait' clause, so this
reuses all of that infrastructure. So all this has to do is support a
pair of clauses that are already implemented (if and async), plus create
an AST node.  This patch does so, and adds proper testing.

This is a clause that is only valid on 'host_data' constructs, and
identifies variables which it should use the current device address.
From a Sema perspective, the only thing novel here is mild changes to
how ActOnVar works for this clause, else this is very much like the rest
of the 'var-list' clauses.

'delete' is another clause that has very little compile-time
implication, but needs a full AST that takes a var list.  This patch
ipmlements it fully, plus adds sufficient test coverage.

This is another new clause specific to 'exit data' that takes a pointer
argument. This patch implements this the same way we do a few other
clauses (like attach) that have the same restrictions.

The 'if_present' clause controls the replacement of addresses in the
var-list in current device memory.  This clause can only go on
'host_device'.  From a Sema perspective, there isn't anything to do
beyond add this to AST and pass it on.