[AutoDiff] Skip implicit GenericTypeParamDecl AST verification. (#32343)

The differentiation transform creates implicit auxiliary struct and enum
declarations, cloning implicit `GenericTypeParamDecl`s from a generic
signature into a flat `GenericParamList`.

These `GenericTypeParamDecl`s may fail depth verification based on
`DC->getGenericContextDepth()` because the auxiliary struct and enum
declarations are always flat.

Skipping verification for implicit `GenericTypeParamDecl`s created by the
differentiation transform is the easiest way to fix errors. No other
code paths create implicit `GenericTypeParamDecl`s.

An alternative is to make the differentiation transform properly generated
nested struct and enum declarations. This should fix verification but
bloats the number of generated declarations.

Author: dan-zheng

lib/AST/ASTVerifier.cpp

@@ -2764,6 +2764,11 @@ class Verifier : public ASTWalker {
         abort();
       }
 
+      // Skip implicit generic param decls. Their depth and index may not be
+      // consistent with the generic context's parameter list.
+      if (GTPD->isImplicit())
+        return;
+
       unsigned currentDepth = DC->getGenericContextDepth();
       if (currentDepth < GTPD->getDepth()) {
         Out << "GenericTypeParamDecl has incorrect depth\n";