[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/Frontend/ChainedIncludesSource.cpp b/clang/lib/Frontend/ChainedIncludesSource.cpp
index cb260b4..be99b78 100644
--- a/clang/lib/Frontend/ChainedIncludesSource.cpp
+++ b/clang/lib/Frontend/ChainedIncludesSource.cpp
@@ -43,6 +43,7 @@
   Selector GetExternalSelector(uint32_t ID) override;
   uint32_t GetNumExternalSelectors() override;
   Stmt *GetExternalDeclStmt(uint64_t Offset) override;
+  CXXCtorInitializer **GetExternalCXXCtorInitializers(uint64_t Offset) override;
   CXXBaseSpecifier *GetExternalCXXBaseSpecifiers(uint64_t Offset) override;
   bool FindExternalVisibleDeclsByName(const DeclContext *DC,
                                       DeclarationName Name) override;
@@ -232,6 +233,10 @@
 ChainedIncludesSource::GetExternalCXXBaseSpecifiers(uint64_t Offset) {
   return getFinalReader().GetExternalCXXBaseSpecifiers(Offset);
 }
+CXXCtorInitializer **
+ChainedIncludesSource::GetExternalCXXCtorInitializers(uint64_t Offset) {
+  return getFinalReader().GetExternalCXXCtorInitializers(Offset);
+}
 bool
 ChainedIncludesSource::FindExternalVisibleDeclsByName(const DeclContext *DC,
                                                       DeclarationName Name) {