The optional single template argument for built-in types is not documented in LLVM's comments that list the grammar, but it is [intentionally implemented](https://github.com/llvm/llvm-project/commit/4bc4d51c18d5c087dfbdad1753c84bba8dbf3be0). PiperOrigin-RevId: 606843797 Change-Id: Ib5ef1983812e9ea68a39fe64b87b6ad6ce81b93c
