[WHLSL] ParseConstexpr is very general

[Myles C. Maxfield]

It's called in a few places:

vector<float, 4> // that "4" is a constexpr
float[10] // that "10" is a constexpr

enum asdf : int {
    XYZ = -3 // that "-3" is a constexpr
}

switch (foo) {
    case MyEnum.XYZ: // that "MyEnum.XYZ" is a constexpr
}

Because of all that, parseConstexpr can return something like operator-((x + y).z)

We should do a bunch of filtering to make sure the right kind of constexprs are allowed in the various places where they are allowed.

[Myles C. Maxfield]

Migrated to https://github.com/gpuweb/WHLSL/issues/79