[clang][Modules] Clarify error message when size check fails in lookupModuleFile

(cherry picked from commit 3c9027c)

Author: cyndyishida

clang/lib/Serialization/ModuleManager.cpp

@@ -134,7 +134,9 @@ ModuleManager::addModule(StringRef FileName, ModuleKind Type,
   // Note: ExpectedSize and ExpectedModTime will be 0 for MK_ImplicitModule
   // when using an ASTFileSignature.
   if (lookupModuleFile(FileName, ExpectedSize, ExpectedModTime, Entry)) {
-    ErrorStr = "module file out of date";
+    ErrorStr = IgnoreModTime
+                   ? "module file has a different size than expected"
+                   : "module file has a different size or mtime than expected";
     return OutOfDate;
   }
 

clang/test/Modules/explicit-build.cpp

@@ -199,6 +199,6 @@
 // RUN:            -fmodule-file=%t/c.pcm \
 // RUN:            %s -DHAVE_A -DHAVE_B -DHAVE_C 2>&1 | FileCheck --check-prefix=CHECK-MISMATCHED-B %s
 //
-// CHECK-MISMATCHED-B:      fatal error: module file '{{.*}}b.pcm' is out of date and needs to be rebuilt: module file out of date
+// CHECK-MISMATCHED-B:      fatal error: module file '{{.*}}b.pcm' is out of date and needs to be rebuilt: module file has a different size than expected
 // CHECK-MISMATCHED-B-NEXT: note: imported by module 'c'
 // CHECK-MISMATCHED-B-NOT:  note: