Remove assumption that methods associated to concrete (non-interface)
types have a fully set up signature. Such methods are found through
LookupFieldOrMethod or lookupMethod, or indexed method access from
a Named type. Make sure that the method's signature is type-checked
before use in those cases.

(MethodSets also hold methods but the type checker is not using
them but for internal verification. API clients will be using it
after all methods have been type-checked.)

Some functions such as MissingMethod may now have to type-check a
method and for that they need a *Checker. Add helper functions as
necessary to provide the additional (receiver) parameter but permit
it to be nil if the respective functions are invoked through the API
(at which point we know that all methods have a proper signature and
thus we don't need the delayed type-check).

Since all package-level objects eventually are type-checked through
the top-level loop in Checker.packageObjects we are guaranteed that
all methods will be type-checked as well.

Updates #23203.
Updates #26854.

Change-Id: I6e48f0016cefd498aa70b776e84a48215a9042c5
Reviewed-on: https://go-review.googlesource.com/c/139425Reviewed-by: Alan Donovan <adonovan@google.com>