Switch assert(0/false) llvm_unreachable.

llvm-svn: 140367
diff --git a/clang/lib/Basic/DiagnosticIDs.cpp b/clang/lib/Basic/DiagnosticIDs.cpp
index ace92ac..f7d1470 100644
--- a/clang/lib/Basic/DiagnosticIDs.cpp
+++ b/clang/lib/Basic/DiagnosticIDs.cpp
@@ -494,7 +494,7 @@
   bool ShouldEmitInSystemHeader = false;
 
   switch (MappingInfo & 7) {
-  default: assert(0 && "Unknown mapping!");
+  default: llvm_unreachable("Unknown mapping!");
   case diag::MAP_IGNORE:
     if (Diag.EnableAllWarnings) {
       // Leave the warning disabled if it was explicitly ignored.