Fix handling of destructor names that name typedefs.

1) Fix a regression in llvmorg-11-init-2485-g0e3a4877840 that would
reject some cases where a class name is shadowed by a typedef-name
causing a destructor declaration to be rejected. Prefer a tag type over
a typedef in destructor name lookup.

2) Convert the "type in destructor declaration is a typedef" error to an
error-by-default ExtWarn to allow codebases to turn it off. GCC and MSVC
do not enforce this rule.