This was causing us to emit diagnostics talking about τ_m_n, which is
not helpful.
Now that generic function types print sanely, print them in a few
places where we were previously printing PolymorphicFunctionTypes.
* [FixCode] Add @escaping when overriding mismatch is because of it. rdar://27814862
With the change of default escaping behavior, users' existing code overriding
objc functions may need to add @escaping to make the overriding match as before. This
patch checks if an overriding mismatch is due to the lacking of @escaping and add
it as a fixit.
* [test] Update existing test. NFC
Otherwise, we'll treat lookups on generic parameters as ambiguous,
since we don't have a concrete nominal type to use (coincidentally)
as the "base" of each lookup result.
It's possible we can get rid of this condition altogether, but I don't
want to rock the boat for unqualified lookup right now.
rdar://problem/23087787
