Improve the helpfulness of an assert message

[richard.barton.arm]

Summary

There are two asserts that can trigger if the number of diagnostic messages in a
kind overflows the number of available enums values. The one that fires if you
exceed this limit by one is helpful. The one that fires if you exceed it by more than
one is not as helpful. Combining the two makes sense I think.

[richard.barton.arm]

Abandoning as cfe-commits was not on subscribers

lib/Basic/DiagnosticIDs.cpp

 /// GetDiagInfo - Return the StaticDiagInfoRec entry for the specified DiagID,
 /// or null if the ID is invalid.
 static const StaticDiagInfoRec *GetDiagInfo(unsigned DiagID) {
   // If assertions are enabled, verify that the StaticDiagInfo array is sorted.
 #ifndef NDEBUG
   static bool IsFirst = true; // So the check is only performed on first call.
   if (IsFirst) {
     for (unsigned i = 1; i != StaticDiagInfoSize; ++i) {
-      assert(StaticDiagInfo[i-1].DiagID != StaticDiagInfo[i].DiagID &&
+      assert(StaticDiagInfo[i-1].DiagID < StaticDiagInfo[i].DiagID &&
              "Diag ID conflict, the enums at the start of clang::diag (in "
              "DiagnosticIDs.h) probably need to be increased");
-
-      assert(StaticDiagInfo[i-1] < StaticDiagInfo[i] &&
-             "Improperly sorted diag info");
     }
     IsFirst = false;
   }
 #endif
 
   // Out of bounds diag. Can't be in the table.
   using namespace diag;
   if (DiagID >= DIAG_UPPER_LIMIT || DiagID <= DIAG_START_COMMON)