Make ExprList initializable by result builder

[fwcd]

Since the Array: ExpressibleAsExprList conformance is unlikely to be added due to type system limitations, here is a small patch to make expression lists a bit more convenient to use by making them builder-initializable (currently only affects SequenceExpr and YieldList).

Using this we can shorten

SequenceExpr(elements: ExprList {
  ...
})

to

SequenceExpr {
  ...
}

[fwcd]

@swift-ci please test