C++ PATCH for c++/78948 (instantiation from discarded statement)

[Jason Merrill <jason at redhat dot com>]

P0292 defines the notion of "discarded statement" which is almost but
not quite the same as "unevaluated operand".  This PR shows a case
where we need to be able to tell that we're in a discarded statement
at a lower level than in the parser, so this patch moves the
information about being in a discarded statement from the parser into
saved_scope.  I've also added a test for a couple of cases that
demonstrate why we can't just use cp_unevaluated_context.

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

Attachment: 78948.diff
Description: Text document