if a JSSpecialPropertyDeclaration has a JSDoc type, use it

[rbuckton]

For a JSSpecialPropertyDeclaration (e.g. this.x = y) in a JavaScript file, we currently create a union of the types of all of the locations of the declaration, where each type is either the type from a JSDocTypeTag or the type of the expression. In this change, we treat the JSDocTypeTag in a fashion similar to a type annotation:

• Always use the type of the first declaration with a JSDocTypeTag (if present).
• Subsequent declarations with a JSDocTypeTag must have an identical type.
• If no declaration has a JSDocTypeTag, we follow the existing behavior and create a union of types of the expressions on the right-hand side.

Fixes #15707