C++ PATCH for more N4268 bits, constant evaluation of all non-type args

[Jason Merrill <jason at redhat dot com>]

My earlier patch for N4268 didn't implement the notion of converted
constant-expression, and as a result we weren't properly using
constexpr user-defined conversions to produce values for template
arguments.

Always performing these conversions led to some issues with
convert_nontype_argument getting confused by the trees built up to
express the conversion, especially for pointers to members, where we
would immediately decay the PTRMEM_CST to an INTEGER_CST even when
converting to the same type.  Changing that ended up requiring a
variety of other changes in pointer-to-member handling.

Tested x86_64-pc-linux-gnu, applying to trunk.

Attachment: converted-const.diff
Description: Text document