Dealing with primary-expression tags in the model XML

[Josh Petrie]

The big blocking issue for us right now in v2 is parsing D3D11;
specifically, parsing enumerations from the C++ headers that aren't
simple numeric literals (for example, "foo | bar | baz"). These show
up as <primary-expression> elements in the model XML, with a bunch of
<token> and <and-expression> type children.

Mike, I recall you mentioning you had a solution to this, but I
haven't seen anything checked in related to it -- are you still
working on it / what was it? I was thinking we should just build a way
to parse primary-expression tags into strings we can emit -- I guess
ideally the parser wouldn't even produce the expression tree for us
and would just give us a single element with the expression, but that
may not be viable.

We may need to do some work to handle these expressions again when we
get to the point where we're supporting renaming of all our interface
elements, but we can worry about that later I think.

-- jmp