PR44978: Accept as an extension some cases where destructor name lookup

is ambiguous, but only one of the possible lookup results could possibly
be right.

Clang recently started diagnosing ambiguity in more cases, and this
broke the build of Firefox. GCC, ICC, MSVC, and previous versions of
Clang all accept some forms of ambiguity here (albeit different ones in
each case); this patch mostly accepts anything any of those compilers
accept.