Switch assert(0/false) llvm_unreachable.

llvm-svn: 140367
diff --git a/clang/lib/Frontend/TextDiagnosticBuffer.cpp b/clang/lib/Frontend/TextDiagnosticBuffer.cpp
index 069c86d..c03ac54 100644
--- a/clang/lib/Frontend/TextDiagnosticBuffer.cpp
+++ b/clang/lib/Frontend/TextDiagnosticBuffer.cpp
@@ -26,7 +26,8 @@
   llvm::SmallString<100> Buf;
   Info.FormatDiagnostic(Buf);
   switch (Level) {
-  default: assert(0 && "Diagnostic not handled during diagnostic buffering!");
+  default: llvm_unreachable(
+                         "Diagnostic not handled during diagnostic buffering!");
   case Diagnostic::Note:
     Notes.push_back(std::make_pair(Info.getLocation(), Buf.str()));
     break;