[clang-tidy] Fix cppcoreguidelines-pro-type-vararg false positives with __builtin_ms_va_list

This commit fixes cppcoreguidelines-pro-type-vararg false positives on
'char *' variables.

The incorrect warnings generated by clang-tidy can be illustrated with
the following minimal example:

```
goid foo(char* in) {
  char *tmp = in;
}
```

The problem is that __builtin_ms_va_list desugared as 'char *', which
leads to false positives.

Fixes bugzilla issue 48042.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D101259