[clang] Fix #174158: Overly strict warning ‘Unrecognized parameter passing direction’ (PR #174245)

[Roberto Turrado Camblor via cfe-commits]

rturrado wrote:

> The doxygen docs also allow a space instead of the comma, so enumerating every possible combination in the warning message is perhaps overkill.

I see a conflict there with the current LLVM implementation. We have, for example, these 2 tests checking precisely that whitespaces are not allowed in parameter passing direction:

```
// expected-warning at +1 {{whitespace is not allowed in parameter passing direction}}
/// \param [ in ] a Blah blah.
int test_param8(int a);

// expected-warning at +1 {{whitespace is not allowed in parameter passing direction}}
/// \param [in, out] a Blah blah.
int test_param9(int a);
```

https://github.com/llvm/llvm-project/pull/174245