[clang] p2085 out-of-class comparison operator defaulting

This implements p2085, allowing out-of-class defaulting of comparison
operators, primarily so they need not be inline, IIUC intent. this was
mostly straigh forward, but required reimplementing
Sema::CheckExplicitlyDefaultedComparison, as now there's a case where
we have no a priori clue as to what class a defaulted comparison may
be for. We have to inspect the parameter types to find out. Eg:

class X { ... };
bool operator==(X, X) = default;

Thus reimplemented the parameter type checking, and added 'is this a
friend' functionality for the above case.

Reviewed By: mizvekov

Differential Revision: https://reviews.llvm.org/D104478