Complete the switch from mappings to declarative diagnostic severities

This begins to address cognitive dissonance caused by treating the Note
diagnostic level as a severity in the diagnostic engine.

No change in functionality.

llvm-svn: 210758
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index cbb6401..855991d 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -2542,7 +2542,7 @@
              I = point.State->begin(), E = point.State->end(); I != E; ++I) {
         if (I->second.isPragma()) {
           Record.push_back(I->first);
-          Record.push_back(I->second.getSeverity());
+          Record.push_back((unsigned)I->second.getSeverity());
         }
       }
       Record.push_back(-1); // mark the end of the diag/map pairs for this