[CodeCompletion] Delete expr type state after getting expr completions

[bitjammer]

When we are getting completions for an initializer at the open
parenthesis, as in:

class C {
func foo<S: Sequence>(x: S) {
String(#^A^#
}
}

after getting all of the overloads for String.init or other applicable
completions for the expression, we leave the stateful expression type
set when performing the last part of code completion, which is getting
other visible declarations at that point.

In this example, C.foo is available to call. However, if the expression
type is left around, we will mistakenly try to use it to substitute
generics of the found declaration, which doesn't make sense, because
foo is a method on C, not String in this case.

We really need to make this part of the compiler less stateful in
the future, or at least formalize the state changes more. It might
also make sense to further separate different kinds of completions
and the mechanisms for getting types, as we reuse the same machinery
for methods and module functions, making a lot of fallback assumptions.

rdar://problem/30137466

[bitjammer]

@swift-ci Please test