Make tag declarations redeclarable. This change has three purposes:

  1) Allow the Index library (and any other interested client) to walk
  the set of declarations for a given tag (enum, union, class,
  whatever). At the moment, this information is not readily available.

  2) Reduce our dependence on TagDecl::TypeForDecl being mapped down
  to a TagType (for which getDecl() will return the tag definition, if
  one exists). This property won't exist for class template partial
  specializations.

  3) Make the canonical declaration of a TagDecl actually canonical,
  e.g., so that it does not change when the tag is defined.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@77523 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/PCHReaderDecl.cpp b/lib/Frontend/PCHReaderDecl.cpp
index 99a1a16..65da7dc 100644
--- a/lib/Frontend/PCHReaderDecl.cpp
+++ b/lib/Frontend/PCHReaderDecl.cpp
@@ -112,6 +112,8 @@
 
 void PCHDeclReader::VisitTagDecl(TagDecl *TD) {
   VisitTypeDecl(TD);
+  TD->setPreviousDeclaration(
+                        cast_or_null<TagDecl>(Reader.GetDecl(Record[Idx++])));
   TD->setTagKind((TagDecl::TagKind)Record[Idx++]);
   TD->setDefinition(Record[Idx++]);
   TD->setTypedefForAnonDecl(