[dcl.init]/6 Merged wording with CWG2196, and added references to [basic.types].

Also fixes CWG1728.

Also fixes CWG2277.

Also revert "[class.union] Only standard-layout unions are
pointer-interconvertible" (0611331) which editorially resolved the same
confilct between the non-normative and normative wording in the opposite
direction, by changing the non-normative wording to match the normative
wording.

…cial member functions

Also fixes CWG943, CWG1076

[cpp.subst] Replace "is" with "would be" when talking about a
hypothetical alternative replacement list.

Fixes #1552

Reverse order of "For each constructor of C, if C is defined," to
"If C is defined, for each constructor of C," for clarity and to
parallel adjacent "If C is not defined" bullet.

The introduction of 'concept' to two lists of entities in [basic] was
omitted, since these lists already include the more general 'template'.

[expr.prim.req.compound] Fix example to not be ill-formed due to an
unsatisfiable concept.

…hrasing.

…mple just said.

…ion.

When the template-head is followed by a concept-definition, it is not
technically followed by a declaration, so avoid claiming that it is.

add missing paragraph numbers.

…rom the prior example.

we've branched C++17.