[C++20] [Modules] Warn if we found #include <filename> in module purview (#69555)

Close #68615.

It is generally wrong to include <filename> in the module purview.
Although there are cases to include files in the module purview,
generally these use cases should include files by quotes instead of by
angles. Here we think the files got included by angles are the system
headers.

This is consistency with MSVC too:
https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warnings-by-compiler-version?view=msvc-170#warnings-introduced-in-visual-studio-2022-version-170-compiler-version-1930

Author: ChuanqiXu9

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -922,6 +922,11 @@ def err_header_import_semi_in_macro : Error<
 def err_header_import_not_header_unit : Error<
   "header file %0 (aka '%1') cannot be imported because "
   "it is not known to be a header unit">;
+def warn_pp_include_angled_in_module_purview : Warning<
+  "'#include <filename>' attaches the declarations to the named module '%0'"
+  ", which is not usually intended; consider moving that directive before "
+  "the module declaration">,
+  InGroup<DiagGroup<"include-angled-in-module-purview">>;
 
 def warn_header_guard : Warning<
   "%0 is used as a header guard here, followed by #define of a different macro">,

clang/lib/Lex/PPDirectives.cpp

@@ -2537,6 +2537,10 @@ Preprocessor::ImportAction Preprocessor::HandleHeaderIncludeOrImport(
     return {ImportAction::None};
   }
 
+  if (isAngled && isInNamedModule())
+    Diag(FilenameTok, diag::warn_pp_include_angled_in_module_purview)
+        << getNamedModuleName();
+
   // Look up the file, create a File ID for it.
   SourceLocation IncludePos = FilenameTok.getLocation();
   // If the filename string was the result of macro expansions, set the include

clang/test/Preprocessor/include-in-module-purview.cppm

@@ -0,0 +1,60 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -E -P -I%t -o %t/tmp 2>&1 | FileCheck %t/a.cppm
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -E -P -I%t -o - 2>&1 \
+// RUN:     -Wno-include-angled-in-module-purview | FileCheck %t/a.cppm --check-prefix=CHECK-NO-WARN
+
+//--- a.h
+// left empty
+
+//--- b.h
+#include <stddef.h>
+// The headers not get included shouldn't be affected.
+#ifdef WHATEVER
+#include <stdint.h>
+#endif
+
+//--- a.cppm
+module;
+#include <stddef.h>
+#include <a.h>
+#include <b.h>
+#include "a.h"
+#include "b.h"
+export module a;
+
+#include <stddef.h>
+#include <a.h>
+#include <b.h>
+#include "a.h"
+#include "b.h"
+
+// CHECK: a.cppm:9:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: a.cppm:10:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: a.cppm:11:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: In file included from {{.*}}/a.cppm:11
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: In file included from {{.*}}/a.cppm:13
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+
+module :private;
+#include <stddef.h>
+#include <a.h>
+#include <b.h>
+#include "a.h"
+#include "b.h"
+
+// CHECK: a.cppm:24:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: a.cppm:25:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: a.cppm:26:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: In file included from {{.*}}/a.cppm:26
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: In file included from {{.*}}/a.cppm:28
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+
+// We should have catched all warnings.
+// CHECK: 10 warnings generated.
+
+// CHECK-NO-WARN-NOT: warning