[AutoDiff] Initial support for differentiation of throwing functions

[asl]

This adds initial support for differentiation of functions that may produce Error result. Essentially we wrap the pullback into Optional and emit a diamond-shape control flow pattern depending on whether the pullback value is available or not. VJP emission was modified to accommodate for this. In addition to this, some additional tricks are required as try_apply result is not available in the instruction parent block, it is available in normal successor basic block.

As a result we can now:

• differentiate an active try_apply result (that would be produced from do ... try .. catch constructions)
• try_apply when error result is unreachable (usually try! and similar source code constructs)
• Support (some) throwing functions with builtin differentiation operators. stdlib change will follow. Though we cannot support typed throws here (yet)
• Correctly propagate error types during currying around differentiable functions as well as type-checking for @derivative(of:) attribute, so we can register custom derivatives for functions producing error result
• Added custom derivative for Optional.?? operator (note that support here is not yet complete as we cannot differentiate through autoclosures, so x ?? y works only if y is not active, e.g. a constant value).

Some fixes here and there

[asl]

Tagging @JaapWijnen @kovdan01

[asl]

@swift-ci please test

[asl]

@swift-ci please test

[asl]

Windows fails in unrelated IRGen/async/run-call-void-throws-to-int-throwing_call-sync-nothrow_call-async-throw.sil

@swift-ci please test windows platform

[asl]

@swift-ci please test

[asl]

@swift-ci please test

[kovdan01]

LGTM, thanks! Functionally this is totally OK, but please consider addressing minor suggestions I've left in comments before merging this. Feel free to ignore though.

[asl]

@swift-ci please test

[kovdan01]

@swift-ci please test Windows platform

[asl]

@swift-ci please test Windows platform