Fix a bug in the LLParser where we failed to diagnose landingpads with...

Fix a bug in the LLParser where we failed to diagnose landingpads with non-constant clause operands.

Fixing this also exposed a related issue where the landingpad under construction was not
cleaned up when an error was raised, which would cause bad reference errors before the
error could actually be printed.

llvm-svn: 231634