[clang] fix definition data not being propagated to all redecls (#157019)

This fixes the workaround added in 8a63989, so that when a fake
definition data is corrected, all redeclarations are also updated to
point to it.

Since this regression was never released, there are no release notes.

Fixes #154840

Author: mizvekov

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -2107,6 +2107,8 @@ void ASTDeclMerger::MergeDefinitionData(
     auto *Def = DD.Definition;
     DD = std::move(MergeDD);
     DD.Definition = Def;
+    while ((Def = Def->getPreviousDecl()))
+      cast<CXXRecordDecl>(Def)->DefinitionData = &DD;
     return;
   }
 

clang/test/Modules/GH154840.cpp

@@ -0,0 +1,97 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -fmodule-name=A -fno-cxx-modules -emit-module -fmodules -xc++ A.cppmap -o A.pcm
+// RUN: %clang_cc1 -fmodule-name=B -fno-cxx-modules -emit-module -fmodules -xc++ B.cppmap -o B.pcm -fmodule-file=A.pcm
+// RUN: %clang_cc1 -fmodule-name=C -fno-cxx-modules -emit-module -fmodules -xc++ C.cppmap -o C.pcm -fmodule-file=A.pcm
+// RUN: %clang_cc1 -fmodule-name=D -fno-cxx-modules -emit-module -fmodules -xc++ D.cppmap -o D.pcm -fmodule-file=A.pcm
+// RUN: %clang_cc1 -fmodule-name=E -fno-cxx-modules -emit-module -fmodules -xc++ E.cppmap -o E.pcm -fmodule-file=D.pcm -fmodule-file=B.pcm -fmodule-file=C.pcm
+// RUN: %clang_cc1 -fno-cxx-modules -fmodules -fmodule-file=B.pcm -fmodule-file=E.pcm -emit-llvm -o /dev/null S.cpp
+
+//--- A.h
+namespace std {
+
+template <class T> void zz(T);
+
+template <class> struct vec {
+  struct w {};
+  struct xx {};
+
+  vec(vec &) { init(); }
+  constexpr vec &operator=(const vec &);
+  template <class U> constexpr void pb(U);
+  constexpr void init();
+
+  w s;
+};
+
+template <class T> constexpr void vec<T>::init() {
+  xx yy;
+  zz(yy);
+}
+
+template <class T> constexpr vec<T> &vec<T>::operator=(const vec &) {
+  pb(s);
+  return *this;
+}
+
+template <class T> template <class U> constexpr void vec<T>::pb(U) { init(); }
+} // namespace std
+
+//--- A.cppmap
+module "A" {
+  header "A.h"
+}
+
+//--- X.h
+#pragma clang module import A
+
+namespace project {
+  class thing : std::vec<thing> {};
+} // namespace project
+
+//--- B.h
+#include "X.h"
+
+//--- B.cppmap
+module "B" {
+  header "B.h"
+}
+
+//--- C.h
+#include "X.h"
+
+//--- C.cppmap
+module "C" {
+  header "C.h"
+}
+
+//--- D.h
+#include "X.h"
+
+//--- D.cppmap
+module "D" {
+  header "D.h"
+}
+
+//--- Y.h
+#include "X.h"
+struct other {
+  other() : data(data) {}
+  std::vec<project::thing> data;
+};
+
+//--- E.h
+#include "Y.h"
+
+//--- E.cppmap
+module "E" {
+  header "E.h"
+}
+
+//--- S.cpp
+#pragma clang module import A
+#pragma clang module import E
+void func(std::vec<project::thing> *a, std::vec<project::thing> *b) { *a = *b; }