[Modules] Incorrect ODR detection for unresolved using type

Implement `getUnresolvedUsingType()` and don't create a new
`UnresolvedUsingType` when there is already canonical declaration.

This solve

[Modules] Incorrect ODR detection for unresolved using type

Implement `getUnresolvedUsingType()` and don't create a new
`UnresolvedUsingType` when there is already canonical declaration.

This solved an incorrect ODR detection in modules for uresolved using
type.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D115792

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