Issue Description:

Hitting 'tab' to autocomplete in the REPL environment produced an assertion error after I had filled in the first argument to String.insert. I was hoping that I would get a list of argument labels.

1> var y = ["a", "b", "c"]
y: [String] = 3 values {
[0] = "a"
[1] = "b"
[2] = "c"
}
2> y.insert( "quux", lldb: /home/buildnode/disk2/workspace/oss-swift-3.0-package-linux-ubuntu-16_04/swift/lib/AST/Type.cpp:862: SmallVector<swift::CallArgParam, 4> swift::decomposeArgType(swift::Type, ArrayRef<swift::Identifier>): Assertion `argumentLabels.size() == 1' failed.
Aborted (core dumped)