Fix GCC's pedantic return-type warning -- this enum is fully covered.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154399 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Parse/ParseDecl.cpp b/lib/Parse/ParseDecl.cpp
index 969e0b2..d2ec7aa 100644
--- a/lib/Parse/ParseDecl.cpp
+++ b/lib/Parse/ParseDecl.cpp
@@ -937,6 +937,7 @@
       << SourceRange(BeginLoc, EndLoc);
     return true;
   }
+  llvm_unreachable("All cases handled above.");
 }
 
 void Parser::DiagnoseProhibitedAttributes(ParsedAttributesWithRange &attrs) {