[clang][deps] Teach dep directive scanner about #pragma clang system_header

This ensures we get the correct FileCharacteristic during scanning. In a
yet-to-be-upstreamed branch this fixes observable failures, but it's
also good to handle this on principle: the FileCharacteristic is a
property of the file that is observable in the scanner, so there is
nothing preventing us from depending on it.

rdar://108627403

Differential Revision: https://reviews.llvm.org/D149777

Author: benlangmuir

clang/include/clang/Lex/DependencyDirectivesScanner.h

@@ -68,6 +68,7 @@ enum DirectiveKind : uint8_t {
   pp_pragma_push_macro,
   pp_pragma_pop_macro,
   pp_pragma_include_alias,
+  pp_pragma_system_header,
   pp_include_next,
   pp_if,
   pp_ifdef,

clang/lib/Lex/DependencyDirectivesScanner.cpp

@@ -652,9 +652,22 @@ bool Scanner::lexPragma(const char *&First, const char *const End) {
     return false;
   }
 
-  // #pragma clang.
-  if (!isNextIdentifierOrSkipLine("module", First, End))
+  FoundId = tryLexIdentifierOrSkipLine(First, End);
+  if (!FoundId)
     return false;
+  Id = *FoundId;
+
+  // #pragma clang system_header
+  if (Id == "system_header") {
+    lexPPDirectiveBody(First, End);
+    pushDirective(pp_pragma_system_header);
+    return false;
+  }
+
+  if (Id != "module") {
+    skipLine(First, End);
+    return false;
+  }
 
   // #pragma clang module.
   if (!isNextIdentifierOrSkipLine("import", First, End))

clang/lib/Lex/Lexer.cpp

@@ -4480,6 +4480,7 @@ bool Lexer::LexDependencyDirectiveTokenWhileSkipping(Token &Result) {
     case pp_pragma_push_macro:
     case pp_pragma_pop_macro:
     case pp_pragma_include_alias:
+    case pp_pragma_system_header:
     case pp_include_next:
     case decl_at_import:
     case cxx_module_decl:

clang/unittests/Lex/DependencyDirectivesScannerTest.cpp

@@ -90,7 +90,8 @@ TEST(MinimizeSourceToDependencyDirectivesTest, AllTokens) {
                                            "#pragma pop_macro(A)\n"
                                            "#pragma include_alias(<A>, <B>)\n"
                                            "export module m;\n"
-                                           "import m;\n",
+                                           "import m;\n"
+                                           "#pragma clang system_header\n",
                                            Out, Tokens, Directives));
   EXPECT_EQ(pp_define, Directives[0].Kind);
   EXPECT_EQ(pp_undef, Directives[1].Kind);
@@ -113,7 +114,8 @@ TEST(MinimizeSourceToDependencyDirectivesTest, AllTokens) {
   EXPECT_EQ(pp_pragma_include_alias, Directives[18].Kind);
   EXPECT_EQ(cxx_export_module_decl, Directives[19].Kind);
   EXPECT_EQ(cxx_import_decl, Directives[20].Kind);
-  EXPECT_EQ(pp_eof, Directives[21].Kind);
+  EXPECT_EQ(pp_pragma_system_header, Directives[21].Kind);
+  EXPECT_EQ(pp_eof, Directives[22].Kind);
 }
 
 TEST(MinimizeSourceToDependencyDirectivesTest, EmptyHash) {