[Swift-devel] compile time error handling

[Ben Clifford]

Over the past day or so, I've committed a bunch of compile time error 
handling changes. r1691 is the last one of those for now.

Some more compile error messages will now have source line numbers in 
them.

There is more compile-time static analysis of the program, which should 
result in errors occurring at compile time rather than part-way through 
workflow execution.

Programs which previously ran OK should still compile OK. As always, 
indicate here if not.

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