[ORC] Fix an EDU-update bug in ExecutionSession::IL_failSymbols.

We were catching a local variable, SymMI, by value instead of by reference
during EDU cleanup and this was leaving the dependence graph in an
inconsistent state that could lead to crashes on subsequent emits. Fixing this
bug required us to also avoid aliasing between SymMI and MI (which would have
caused cleanup to clear the MI.DependantEDUs set that we're iterating over).

No testcase: the crash only triggered in very specific circumstances
(including concurrent linking) in an out-of-tree ORC client. I'm working on a
session state verifier that could be turned on when compiling with
expensive-checks turned on and that should help us catch issues like this in
the future.

rdar://125164262

Coding my way home: 0.89527S, 89.61313W