DiagnosticInfo: Allow unsupported be a warning

[arsenm]

Summary

Some unsupported features can be ignored, so don't force this to be a hard error.

[qcolombet]

LGTM

[arsenm]

r273061

include/llvm/IR/DiagnosticInfo.h

 public:
   /// \p Fn is the function where the diagnostic is being emitted. \p DLoc is
   /// the location information to use in the diagnostic. If line table
   /// information is available, the diagnostic will include the source code
   /// location. \p Msg is the message to show. Note that this class does not
   /// copy this message, so this reference must be valid for the whole life time
   /// of the diagnostic.
   DiagnosticInfoUnsupported(const Function &Fn, const Twine &Msg,
-                            DebugLoc DLoc = DebugLoc())
-      : DiagnosticInfoWithDebugLocBase(DK_Unsupported, DS_Error, Fn, DLoc),
+                            DebugLoc DLoc = DebugLoc(),
+                            DiagnosticSeverity Severity = DS_Error)
+      : DiagnosticInfoWithDebugLocBase(DK_Unsupported, Severity, Fn, DLoc),
         Msg(Msg) {}
 
   static bool classof(const DiagnosticInfo *DI) {
     return DI->getKind() == DK_Unsupported;
   }
 
   const Twine &getMessage() const { return Msg; }