clang/Modules: Add missing diagnostics for malformed AST files
These were found via an audit. In the case of ParseLineTable this is
actually dead code, since parsing the line table always succeeds, but
it's prudent to be defensive since it's possible an assertion there
could be converted to a true return in the future.
(cherry picked from commit 8e2c192e2af8c760152ba3b28e774dbb1548e4aa)