Avoid implicit fallthrough in some Parser switches

Summary:
There were a couple parser switch statements that we didn't really need
implicit fallthrough in, and in the TS case it was suboptimal code.

These code paths aren't really exercised right now.

Reviewed By: neildhar

Differential Revision: D58824704

fbshipit-source-id: ded892c0f1ad01e6ce6348c1d5ee303c7a963e68

lib/Parser/JSParserImpl.cpp

@@ -3943,6 +3943,7 @@ Optional<ESTree::Node *> JSParserImpl::parseUnaryExpression() {
             getPrevTokenEndLoc(),
             new (context_) ESTree::TSTypeAssertionNode(*optType, *optExpr));
       }
+      break;
 #endif
 
     case TokenKind::identifier:
@@ -3957,12 +3958,13 @@ Optional<ESTree::Node *> JSParserImpl::parseUnaryExpression() {
             getPrevTokenEndLoc(),
             new (context_) ESTree::AwaitExpressionNode(optExpr.getValue()));
       }
-      // Fall-through to default for all other identifiers.
-      LLVM_FALLTHROUGH;
+      // Default for all other identifiers.
+      break;
 
     default:
-      return parsePostfixExpression();
+      break;
   }
+  return parsePostfixExpression();
 }
 
 namespace {

lib/Parser/rust-api.cpp

@@ -445,7 +445,8 @@ enum class DiagKind : uint32_t {
 DiagKind toDiagKind(llvh::SourceMgr::DiagKind k) {
   switch (k) {
     default:
-      assert(false);
+      assert(false && "Invalid DiagKind");
+      [[fallthrough]];
     case llvh::SourceMgr::DK_Error:
       return DiagKind::Error;
     case llvh::SourceMgr::DK_Warning: