[clang-tools-extra] [clang-tidy] Fix readability-identifier-naming FP on function templates (PR #198517)

[via cfe-commits]

CarvedCoder wrote:

The forward declarations for structs and union doesn't seem to be working too 

`
warning: invalid case style for identifier 'MyClass' [readability-identifier-naming]
    1 | class MyClass;
      |       ^~~~~~~
      |       my_class
 warning: invalid case style for identifier 'MyStruct' [readability-identifier-naming]
    5 | struct MyStruct;
      |        ^~~~~~~~
      |        my_struct
warning: invalid case style for identifier 'MyUnion' [readability-identifier-naming]
    9 | union MyUnion;
      |       ^~~~~~~
      |       my_union

` 

when

`
Checks: "-*,readability-identifier-naming"

CheckOptions:
  - key: readability-identifier-naming.DefaultCase
    value: lower_case

  - key: readability-identifier-naming.ClassCase
    value: CamelCase

  - key: readability-identifier-naming.StructCase
    value: CamelCase

  - key: readability-identifier-naming.UnionCase
    value: CamelCase

  - key: readability-identifier-naming.TemplateParameterCase
    value: CamelCase
`

https://github.com/llvm/llvm-project/pull/198517