[clang] extend external_source_symbol attribute with USR clause

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