[CodeCompletion] Using the default constraints generator for TupleExpr.

[nkcsgexi]

[CodeCompletion] Using the default constraints generator for TupleExpr
when inferring the type of unresolved members. rdar://28991372

When code completing, we used to create a type variable to represent the type
of an entire tuple expression. However, recent improvements on parser make this
step unnecessary and crash-prone. Thus, we use the default constraint
generator to interpret tuple expressions.

[nkcsgexi]

@swift-ci please smoke test