Fixes https://github.com/dlang/dmd/issues/20499
Fixes https://github.com/dlang/dmd/issues/20963
ImportC deferred declaring "tagged" types (structs/unions/enums)
until after it saw a possible typedef so that the identifier for
a typedef declaration like:
typedef struct { int x; } Foo;
would give the struct the name Foo. In several circumstances,
this led to tagged types not being declared. Resolve this by
chasing down those circumstances.
Also, there were other circumstances where types weren't being
correctly declared which caused other issues. Lock those down.
