Suppress check for system headers

Author: Jinjie-Huang

clang/lib/Lex/HeaderSearch.cpp

@@ -921,6 +921,9 @@ void HeaderSearch::DiagnoseHeaderShadowing(
  It = FromDir;
  }
  for (; It != search_dir_end(); ++It) {
+ // Suppress check for system headers, as duplicates are often intentional.
+ if (It->getDirCharacteristic() != SrcMgr::C_User)
+ continue;
  SmallString<1024> TmpPath = It->getName();
  llvm::sys::path::append(TmpPath, Filename);
  if (auto File = getFileMgr().getFileRef(TmpPath, false, false)) {

clang/test/Preprocessor/header-shadowing.c

@@ -10,10 +10,10 @@
 
 // SHADOWING: {{.*}} warning: multiple candidates for header 'header.h' found; directory '{{.*}}include1' chosen, ignoring '{{.*}}include2' and others [-Wshadow-header]
 // SHADOWING: warning: include1/header.h included!
-// SHADOWING: warning: include2/header.h included!
-// SHADOWING: warning: system1/stdio.h included!
 // SHADOWING-NOT: {{.*}} warning: multiple candidates for header 'header.h' found; directory '{{.*}}include2' chosen, ignoring '{{.*}}include1' and others [-Wshadow-header]
+// SHADOWING: warning: include2/header.h included!
 // SHADOWING-NOT: {{.*}} warning: multiple candidates for header 'stdio.h' found; directory '{{.*}}system1' chosen, ignoring '{{.*}}system2' and others [-Wshadow-header]
+// SHADOWING: warning: system1/stdio.h included!
 
 /// Check that the diagnostic is only performed once in MSVC compatibility mode.
 // RUN: %clang_cc1 -fms-compatibility -Wshadow-header -Eonly %t/t.c -I %t -I %t/foo -I %t/foo/bar 2>&1 | FileCheck %s --check-prefix=SHADOWING-MS