[ASTImporter] Fix AST import crash for a friend decl

[vabridgers]

Summary

Running CTU testing, we found that VisitFriendDecl in
ASTImporterLookup.cpp was not handling a particular non-dependent case,
so we reached the llvm_unreachable case.

The FriendDecl and QualType not handled were:

(gdb) p D->dump()
FriendDecl 0x7ffff5cf1958
< <<srcfile>>, 'nlohmann::basic_json<std::map, std::vector,

std::basic_string<char>, bool, long long, unsigned long long, double,
std::allocator, adl_serializer, std::vector<unsigned char,
std::allocator<unsigned char>>>':'nlohmann::basic_json<std::map,
std::vector, std::basic_string<char>, bool, long long, unsigned long
long, double, std::allocator, adl_serializer, std::vector<unsigned char,
std::allocator<unsigned char>>>'

(gdb) p Ty->dump()
SubstTemplateTypeParmType 0x7ffff5cf0df0 'class
nlohmann::basic_json<std::map, std::vector, class

std::basic_string<char>, _Bool, long long, unsigned long long, double,
std::allocator, adl_serializer, class std::vector<unsigned char, class
std::allocator<unsigned char> > >' sugar

-TemplateTypeParmType 0x7ffff643ea40 'BasicJsonType' dependent depth 0

index 0

`-TemplateTypeParm 0x7ffff643e9e8 'BasicJsonType'

`-RecordType 0x1012ad20 'class nlohmann::basic_json<std::map,
std::vector, class std::basic_string<char>, _Bool, long long, unsigned
long long, double, std::allocator, adl_serializer, class
std::vector<unsigned char, class std::allocator<unsigned char> > >'

`-ClassTemplateSpecialization 0x1012ab68 'basic_json'

[vabridgers]

fix pre-merge checks

[martong]

Thanks Vince, looks good to me!

clang/lib/AST/ASTImporterLookupTable.cpp

if (D->getFriendType()) {
       // always has that decl as child node.
       // However, there are non-dependent cases which does not have the
       // type as a child node. We have to dig up that type now.
       if (!Ty->isDependentType()) {
         if (const auto *RTy = dyn_cast<RecordType>(Ty))
           LT.add(RTy->getAsCXXRecordDecl());
         else if (const auto *SpecTy = dyn_cast<TemplateSpecializationType>(Ty))
           LT.add(SpecTy->getAsCXXRecordDecl());
-        else if (isa<TypedefType>(Ty)) {
+        else if (const auto *SubstTy =
+                     dyn_cast<SubstTemplateTypeParmType>(Ty)) {
+          if (SubstTy->getAsCXXRecordDecl())
+            LT.add(SubstTy->getAsCXXRecordDecl());
+        } else if (isa<TypedefType>(Ty)) {
           // We do not put friend typedefs to the lookup table because
           // ASTImporter does not organize typedefs into redecl chains.
         } else {
           llvm_unreachable("Unhandled type of friend class");
         }
       }
     }
     return true;