This folds member access on types to a TypeExpr if the member
resolves to a nested type.
This allows [Foo.Bar]() to become an ApplyExpr of a TypeExpr,
rather than an ApplyExpr of an array literal; previously,
only [Foo]() worked.
Two cases that are still unsupported:
1) If G is a generic type and T is a generic typealias, then
G.T<X> cannot be folded to a TypeExpr, because we cannot
represent a generic typealias with an unbound generic
parent type. Such lookups remain member lookups, where
Sema first opens the base type to produce a bound generic
type G<$T0>, and then resolves the generic typealias
member on that.
2) If T is a generic parameter and X is an associated type,
T.X is not folded down to a TypeExpr either.
Fixes <rdar://problem/16849958>.
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.
