Avoid double deletion of the ASTContext's external source.
Avoid double deletion of the ASTContext's external source.

This is another hack, working around the lack of multiplexing external AST source.

karies/cling,root-mirror/cling,perovic/cling,karies/cling,root-mirror/cling,perovic/cling,root-mirror/cling,karies/cling,perovic/cling,marsupial/cling,karies/cling,perovic/cling,marsupial/cling,marsupial/cling,root-mirror/cling,perovic/cling,marsupial/cling,root-mirror/cling,karies/cling,marsupial/cling,karies/cling,marsupial/cling,root-mirror/cling