[C++20][Modules] Relax ODR check in unnamed modules (#111160)

Summary:
Option `-fskip-odr-check-in-gmf` is set by default and I think it is
what most of C++ developers want. But in header units, Clang ODR
checking is too strict, making them hard to use, as seen in the example
in the diff. This diff relaxes ODR checks for unnamed modules to match
GMF ODR checking.

Test Plan: check-clang

Author: dmpolukhin

clang/include/clang/AST/DeclBase.h

@@ -687,6 +687,9 @@ class alignas(8) Decl {
   /// Whether this declaration comes from a named module.
   bool isInNamedModule() const;
 
+  /// Whether this declaration comes from a header unit.
+  bool isFromHeaderUnit() const;
+
   /// Return true if this declaration has an attribute which acts as
   /// definition of the entity, such as 'alias' or 'ifunc'.
   bool hasDefiningAttr() const;

clang/include/clang/Serialization/ASTReader.h

@@ -2527,7 +2527,7 @@ class BitsUnpacker {
 
 inline bool shouldSkipCheckingODR(const Decl *D) {
   return D->getASTContext().getLangOpts().SkipODRCheckInGMF &&
-         D->isFromGlobalModule();
+         (D->isFromGlobalModule() || D->isFromHeaderUnit());
 }
 
 } // namespace clang

clang/lib/AST/DeclBase.cpp

@@ -1168,6 +1168,10 @@ bool Decl::isInNamedModule() const {
   return getOwningModule() && getOwningModule()->isNamedModule();
 }
 
+bool Decl::isFromHeaderUnit() const {
+  return getOwningModule() && getOwningModule()->isHeaderUnit();
+}
+
 static Decl::Kind getKind(const Decl *D) { return D->getKind(); }
 static Decl::Kind getKind(const DeclContext *DC) { return DC->getDeclKind(); }
 

clang/test/Headers/header-unit-common-cmp-cat.cpp

@@ -0,0 +1,40 @@
+// RUN: rm -fR %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: %clang_cc1 -verify -std=c++20 -fskip-odr-check-in-gmf -I. -emit-header-unit -xc++-user-header bz1.h
+// RUN: %clang_cc1 -verify -std=c++20 -fskip-odr-check-in-gmf -I. -emit-header-unit -xc++-user-header bz2.h
+// RUN: %clang_cc1 -verify -std=c++20 -fskip-odr-check-in-gmf -I. -emit-header-unit -xc++-user-header -fmodule-file=bz1.pcm -fmodule-file=bz2.pcm bz.cpp
+
+//--- compare
+namespace std {
+namespace __detail {
+
+template<typename _Tp>
+inline constexpr unsigned __cmp_cat_id = 1;
+
+template<typename... _Ts>
+constexpr auto __common_cmp_cat() {
+  (__cmp_cat_id<_Ts> | ...);
+}
+
+} // namespace __detail
+} // namespace std
+
+//--- bz0.h
+template <class T>
+int operator|(T, T);
+
+//--- bz1.h
+#include "bz0.h"
+#include <compare>
+// expected-no-diagnostics
+
+//--- bz2.h
+#include <compare>
+// expected-no-diagnostics
+
+//--- bz.cpp
+#include <compare>
+
+import "bz1.h"; // expected-warning {{the implementation of header units is in an experimental phase}}
+import "bz2.h"; // expected-warning {{the implementation of header units is in an experimental phase}}