[clang][modules] Correctly set module map systemness

This uses the systemness of the module map instead of of the Module
instance, as doing otherwise could incorrectly parse the other modules
in that module map as system.

This is still correct as the only ways to get a system module are
by the module map being in a system path, or the module having the
[system] attribute, both of which are handled here.

This makes it so that the systemness of a module is deterministic
instead of depending on the path taken to build it.

Author: Bigcheese

clang/lib/Frontend/CompilerInstance.cpp

@@ -1360,10 +1360,17 @@ static bool compileModule(CompilerInstance &ImportingInstance,
 
     StringRef ModuleMapFilePath = ModuleMapFile->getNameAsRequested();
 
+    // Use the systemness of the module map as parsed instead of using the
+    // IsSystem attribute of the module. If the module has [system] but the
+    // module map is not in a system path, then this would incorrectly parse
+    // any other modules in that module map as system too.
+    const SrcMgr::SLocEntry &SLoc = SourceMgr.getSLocEntry(ModuleMapFID);
+    bool IsSystem = isSystem(SLoc.getFile().getFileCharacteristic());
+
     // Use the module map where this module resides.
     Result = compileModuleImpl(
         ImportingInstance, ImportLoc, Module->getTopLevelModuleName(),
-        FrontendInputFile(ModuleMapFilePath, IK, +Module->IsSystem),
+        FrontendInputFile(ModuleMapFilePath, IK, IsSystem),
         ModMap.getModuleMapFileForUniquing(Module)->getName(), ModuleFileName);
   } else {
     // FIXME: We only need to fake up an input file here as a way of

clang/test/Modules/transitive-system.test

@@ -0,0 +1,27 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: sed "s|DIR|%/t|g" %t/cdb.json.template > %t/cdb.json
+// RUN: clang-scan-deps -compilation-database %t/cdb.json -format experimental-full -module-name=direct > %t/result1.json
+// RUN: rm -rf %t/cache
+// RUN: clang-scan-deps -compilation-database %t/cdb.json -format experimental-full -module-name=transitive > %t/result2.json
+// RUN: %deps-to-rsp %t/result1.json --module-name transitive > %t/1.rsp
+// RUN: %deps-to-rsp %t/result2.json --module-name transitive > %t/2.rsp
+// RUN: diff %t/1.rsp %t/2.rsp
+
+//--- module.modulemap
+module direct [system] { header "direct.h" }
+module transitive { header "transitive.h" }
+
+//--- direct.h
+#include "transitive.h"
+
+//--- transitive.h
+// empty
+
+//--- cdb.json.template
+[{
+  "file": "",
+  "directory": "DIR",
+  "command": "clang -fmodules -fmodules-cache-path=DIR/cache -I DIR -x c"
+}]