Make it clear that getSTDIN returns null when stdin is empty.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@40940 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/MemoryBuffer.h

@@ -76,7 +76,7 @@ public:
                                              const char *BufferName = "");
   
   /// getSTDIN - Read all of stdin into a file buffer, and return it.  This
-  /// fails if stdin is empty.
+  /// returns null if stdin is empty.
   static MemoryBuffer *getSTDIN();