[clang] Improve module out of date error message

When a pcm file has a different size or modification time than it had
when it was written to another module's IMPORT table Clang emits:

`<pcm> is out of date and needs to be rebuilt: module file out of date`

This is difficult to understand what's happening because there are a
lot of reasons that a module file can be out of date. This changes
the latter part of that message to:

`module file has a different size or mtime than expected`

Which makes it clearer what the issue is. For future work it would be
nice if a more detailed explanation of the issue could be emitted as a
note instead.

Author: Bigcheese

clang/lib/Serialization/ModuleManager.cpp

@@ -119,7 +119,7 @@ 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-    ErrorStr = "module file out of date";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 = "module file has a different size or mtime than expected";
     return OutOfDate;
   }