[clang] Silence GCC warnings about control reaching end of non void function

This fixes GCC warnings since
e392056.

Author: mstorsjo

clang/include/clang/Basic/DiagnosticIDs.h

@@ -18,6 +18,7 @@
 #include "clang/Basic/LLVM.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/ErrorHandling.h"
 #include <optional>
 #include <vector>
 
@@ -310,6 +311,7 @@ class DiagnosticIDs : public RefCountedBase<DiagnosticIDs> {
         return {diag::Severity::Fatal, std::string(Message), CLASS_ERROR,
                 /*ShowInSystemHeader*/ true};
       }
+      llvm_unreachable("Fully covered switch above!");
     }());
   }
 

clang/lib/Sema/SemaOverload.cpp

@@ -7324,6 +7324,7 @@ static bool diagnoseDiagnoseIfAttrsWith(Sema &S, const NamedDecl *ND,
     case DiagnoseIfAttr::DS_error:
       return diag::Severity::Error;
     }
+    llvm_unreachable("Fully covered switch above!");
   };
 
   for (const auto *DIA : llvm::make_range(WarningBegin, Attrs.end()))