[CodeComplete] Member completion: heuristically resolve some dependent base exprs

Today, inside a template, you can get completion for:

Foo<T> t;
t.^

t has dependent type Foo<T>, and we use the primary template to find its members.
However we also want this to work:

t.foo.bar().^

The type of t.foo.bar() is DependentTy, so we attempt to resolve using similar
heuristics (e.g. primary template).

Differential Revision: https://reviews.llvm.org/D96376