Make QStringLiteral always choke on non-literals

Make QStringLiteral always choke on non-literals

The fallback implementation of QStringLiteral did not (up to now)
enforce the need to use a literal. So it was possible to write:

  const char *foo = "Hello";
  QString s = QStringLiteral(foo);

Which would do the wrong thing and create s == "Hel" on 32-bit
platforms (sizeof(foo) == 4) or, wrose, s == "Hello\0XY" on 64-bit
platforms (sizeof(foo) == 8, X and Y are garbage).

This change enforces the need for a literal by producing errors on the
above cases, as well as when foo is a char array variable.

GCC:
error: expected ‘)’ before ‘foo’

Clang (abbreviated):
error: expected ')'
namespace X { QString x() { const char foo[42] = "Hello"; return QStringLiteral(foo); } }
                                                                                ^
note: to match this '('
                                              ^

ICC:
error: expected a ")"
  namespace X { QString x() { const char foo[42] = "Hello"; return QStringLiteral(foo); } }
                                                                   ^

The first C++11 error currently is:
error: expected primary-expression before ‘enum’  (GCC)
error: expected a ")"   (ICC)

Change-Id: I317173421dbd7404987601230456471c93b122ed
Reviewed-by: hjk <qthjk@ovi.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>