This extends 985dacea by XFAILing more tests that started failing after these commits:
* 14498a477ee9e00dc462779cee8cbc5846ca6d3a
* e644f5973b0b71baadc6d7b64596527a1dc49d17
* 89bacc0bb9f6aa66ca1951ec5c0e4c38cc661160
These failures need to be investigated. Possible cause is D151938.
Allow the user to specify a concrete USR in the external_source_symbol attribute.
That will let Clang's indexer to use Swift USRs for Swift declarations that are
represented with C++ declarations.
This new clause is used by Swift when generating a C++ header representation
of a Swift module:
https://github.com/apple/swift/pull/63002
Differential Revision: https://reviews.llvm.org/D141324