[Bug c++/107279] New: __builtin_complex is not supported

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107279

            Bug ID: 107279
           Summary: __builtin_complex is not supported
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: documentation
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

I noticed this while trying to create a testcase for another issue.
And it is not documented that __builtin_complex is only support for the C
front-end (I know because I touched the documentation last).
testcase:
```
void f(void)
{
  _Complex float a = __builtin_complex (1.0f, 2.0f);
}
```