Add serialization support for SubstNonTypeTemplateParmExpr.

Also add the missing serialization support for SEHTryStmt,
SEHFinallyStmt, and SEHExceptStmt, and fix and finish the
serialization support for AsTypeExpr.  In addition, change
the code so that it will no longer link if a Stmt subclass
is missing serialization support.

llvm-svn: 135258