[clang] [clang]fix ppcaching assertion gh186582 (PR #187370)

[Aaron Ballman via cfe-commits]

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AaronBallman wrote:

```suggestion
a(   ::template operator // expected-error 2 {{expected a type}} \
                            expected-error {{a type specifier is required for all declarations}} \
                            expected-error {{expected ';' after top level declarator}}
```
to make it easier to read and should fix the issue with the CI failure.

https://github.com/llvm/llvm-project/pull/187370