[clang] Fix non-deterministic infinite recursion... (#118288)

[mpusz]

...in ASTContext::getAutoTypeInternal

Given

template < typename >
concept C1 = true;

template < typename , auto >
concept C2 = true;

template < C1 auto V, C2< V > auto>
struct S;

Both C1 auto V and C2<V> auto end on the set AutoType, the former being a template parameter for the latter.

Since the hashing is not deterministic (i.e., pointers are hashed), every now and then, both will end on the same bucket. Given that FoldingSet recomputes the FoldingSetID for each node in the target bucket on lookup, this triggers an infinite recursion:

1. Look for X in AutoTypes
2. Let's assume it would be in bucket N, so it iterates over nodes in that bucket. Let's assume the first is C2<V> auto.
3. Computes the FoldingSetID for this one, which requires the profile of its template parameters, so they are visited.
4. In some frames below, we end on the same FoldingSet, and, by chance, C1 auto V would be in bucket N too.
5. But the first node in the bucket is C2<V> auto for which we need to profile C1 auto V
6. ... stack overflow!

No step individually does anything wrong, but in general, FoldingSet seems not to be re-entrant, and this fact is hidden behind many nested calls.

With this change, we store the AutoTypes inside a DenseMap instead. The FoldingSetID is computed once only and then kept as the map's key, avoiding the need to do recursive lookups.

We also now make sure the key for the inserted AutoType is the same as the key used for lookup. Before, this was not the case, and it caused also non-deterministic parsing errors.

Fixes llvm#110231