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.
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