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Issue Description:

In an @objc class, if you implement subscript taking an Int or object, the Method that gets emitted has a NULL type encoding, which can cause crashes in code that is inspecting the runtime (debug-only in my case, but still annoying to track down and ideally the runtime data would at least have id/NSInteger-based key/return generic signatures.

Tested in swift-2.2-branch tip and in the 3/1 development snapshot.

See the attached sample, which produces:

method = 0x000000010b4a3178
method name = objectAtIndexedSubscript:
method types = 0x0000000000000000
method = 0x000000010b4a3190
method name = setObject:atIndexedSubscript:
method types = 0x0000000000000000
method = 0x000000010b4a31a8
method name = objectForKeyedSubscript:
method types = 0x0000000000000000
method = 0x000000010b4a31c0
method name = setObject:forKeyedSubscript:
method types = 0x0000000000000000