[llvm] [mlir] [ADT] Use a dedicated empty type for StringSet (NFC) (PR #165967)

[Kazu Hirata via llvm-commits]

================
@@ -21,6 +21,9 @@
 
 namespace llvm {
 
+/// The "value type" of StringSet represented as an empty struct.
+struct StringSetTag {};
----------------
kazutakahirata wrote:

Fixed.

https://github.com/llvm/llvm-project/pull/165967