[Bug c/66953] New: Missing documentation for support of unicode literals under -std=gnu99

["jeansaad at ca dot ibm.com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66953

            Bug ID: 66953
           Summary: Missing documentation for support of unicode literals
                    under -std=gnu99
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jeansaad at ca dot ibm.com
  Target Milestone: ---

gcc allows for the use of unicode literals under -std=gnu99 however this
support is not documented under C Extensions
(https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html#C-Extensions). I can only
imagine that this support was intentional as an extended option to the C99
standard in support of TR 19769.

### STANDALONE SOURCE:
#include <uchar.h>

int main(void) {
        char16_t c16 = u'\u00F6';
        char32_t c32 = U'\U0010FFFF';
}

### GCC INVOCATION:
gcc -std=gnu99 -c unicode_literals.c

## gcc -v OUTPUT:
Thread model: posix
gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04)