fix bug that undefined internal is a warning only for -pedantic-errors (#98016)

This fixes issue #39558 which mentions the problem when compiling the
following code with `-pedantic-errors` flag (it also mentions `-Wall
-Wextra` but those shouldn't change the output, which is also the case
in GCC):

```c
static void f();

int main()
{
    f;
}
```

Clang only outputs an `undefined-internal` warning on the first line,
but according to 6.9/3

```
"... Moreover, if an identifier declared with internal linkage is used in an
 expression (other than as a part of the operand of a sizeof or _Alignof
 operator whose result is an integer constant), there shall be exactly one
 external definition for the identifier in the translation unit."
```

this should be illegal and hence an error.

I fixed this by changing the warning type from `Warning` to `ExtWarn`
and by adding a suitable test.

0 files changed, 0 insertions, 0 deletions