Do not always instantiate jsx signatures

[weswigham]

When I changed jsx to actually do inference, I hadn't realized that JSX had been instantiating signatures during its use in the getContextualType code path. Since that path doesn't specify a sourceAttributesType (because we're in the process of calculating it), signatures were being instantiated with {} (in the associated issue, this meant that props was set to {} and then {} was used to contextually type the attributes, rather than P extends { x?: "a" | "b" }). Now, if sourceAttributesType is undefined, we do not instantiate the signatures at all, which is what the contextual typing code actually expects.

Fixes #21028

[weswigham]

Note, the bug @Andy-MS reported (#21028) is responsible for troublesome breaks in dt, so this fix should be release in the initial 2.7 drop, not 2.8.