silence -Wreturn-type warnings

These codepaths would generate warnings with GCC on linux even though the switch
was covered.  Add llvm_unreachable markers to indicate that the switch should be
covered.  NFC.

llvm-svn: 260865
diff --git a/clang/lib/Index/IndexSymbol.cpp b/clang/lib/Index/IndexSymbol.cpp
index bf3bfd1..95ae977 100644
--- a/clang/lib/Index/IndexSymbol.cpp
+++ b/clang/lib/Index/IndexSymbol.cpp
@@ -266,6 +266,7 @@
   case SymbolKind::CXXTypeAlias: return "type-alias";
   case SymbolKind::CXXInterface: return "c++-__interface";
   }
+  llvm_unreachable("invalid symbol kind");
 }
 
 StringRef index::getTemplateKindStr(SymbolCXXTemplateKind TK) {
@@ -275,6 +276,7 @@
   case SymbolCXXTemplateKind::TemplatePartialSpecialization : return "TPS";
   case SymbolCXXTemplateKind::TemplateSpecialization: return "TS";
   }
+  llvm_unreachable("invalid template kind");
 }
 
 StringRef index::getSymbolLanguageString(SymbolLanguage K) {
@@ -283,4 +285,5 @@
   case SymbolLanguage::ObjC: return "ObjC";
   case SymbolLanguage::CXX: return "C++";
   }
+  llvm_unreachable("invalid symbol language kind");
 }