Fix unreachable-fallthrough issue in rust-api.cpp

Summary:
Previously the code just did silent implicit fallthrough in prod mode. I added
`[[fallthrough]]` to make this explicit. However, the code's behaviour still
differs between dev and prod in a way that might compromise correctness.

This diff follows the suggestion of tmikov on D79836435.

Reviewed By: tmikov

Differential Revision: D79966036

fbshipit-source-id: 319b5d89682608360fda23ff5ecb611f17852450

Author: r-barnes

lib/Parser/rust-api.cpp

@@ -446,7 +446,7 @@ DiagKind toDiagKind(llvh::SourceMgr::DiagKind k) {
   switch (k) {
     default:
       assert(false && "Invalid DiagKind");
-      [[fallthrough]];
+      abort();
     case llvh::SourceMgr::DK_Error:
       return DiagKind::Error;
     case llvh::SourceMgr::DK_Warning: