[modules] Deserialize CXXCtorInitializer list for a constructor lazily.

Previously we'd deserialize the list of mem-initializers for a constructor when
we deserialized the declaration of the constructor. That could trigger a
significant amount of unnecessary work (pulling in all base classes
recursively, for a start) and was causing problems for the modules buildbot due
to cyclic deserializations. We now deserialize these on demand.

This creates a certain amount of duplication with the handling of
CXXBaseSpecifiers; I'll look into reducing that next.

llvm-svn: 233052
diff --git a/clang/lib/AST/ExternalASTSource.cpp b/clang/lib/AST/ExternalASTSource.cpp
index 8894107..730842a 100644
--- a/clang/lib/AST/ExternalASTSource.cpp
+++ b/clang/lib/AST/ExternalASTSource.cpp
@@ -66,6 +66,11 @@
   return nullptr;
 }
 
+CXXCtorInitializer **
+ExternalASTSource::GetExternalCXXCtorInitializers(uint64_t Offset) {
+  return nullptr;
+}
+
 CXXBaseSpecifier *
 ExternalASTSource::GetExternalCXXBaseSpecifiers(uint64_t Offset) {
   return nullptr;