[clang] Fix a crash during code completion

During code completion, lookupInDeclContext() calls
CodeCompletionDeclConsumer::FoundDecl(),which can mutate StoredDeclsMap,
over which lookupInDeclContext() iterates. This can lead to invalidation
of iterators and an assert()-crash.

Example code where this happens:
 #include <list>
 int main() {
   std::list<int>;
   std::^
 }
with code completion on ^ with -std=c++20.

I do not have a repro case that does not need standard library.

This fix stores pointers to NamedDecls in a temporary vector, then
visits them outside of the main loop, when StoredDeclsMap iterators are
gone.

Differential Revision: https://reviews.llvm.org/D103472