In an instance declaration when you don't specify the code for a
method, GHC fills in from the default binding in the class.
The type of the default method can legitmiately be ambiguous ---
see Note [Default methods in instances] in TcInstDcls --- so
typechecking it can be tricky.

Trac #12220 showed that although we were dealing with that ambiguity
for /vanilla/ default methods, we were not doing so for /generic/
default methods.  Moreover we were dealing with it clumsily, by
generating post-typechecked code.

This patch fixes the bug AND deletes code!  We now use the same code
path for both vanilla and generic default methods; and generate
/pre-typechecked/ code in both cases.  The key trick is that we can use
Visible Type Application to deal with the ambiguity, which wasn't
possible before.  Hooray.

There is a small hit to performance in compiler/perf/T1969 which
consists of nothing BUT instance declarations with several default
methods to fill, which we now have to typecheck. The actual hit is
from 724 -> 756 or 4% in that extreme example.  Real world programs
have vastly fewer instance decls.