_Bool isn't just a 1-bit integer because when you convert a value
to bool, the result is 1 if the original value is non-zero. This
is contrary to the other small integral types, e.g. char, as you
can see below:

  char x  = 256; // x is 0
  _Bool y = 256; // y is 1

This patch allows writing a comma expression on the left-hand side
of an assignment expression. This is called the "generalized lvalue"
which is a deprecated GCC language extension. I'm implementing it
anyway because it's useful to implement other features.

No functionality change

This is a GNU C extension but will be useful for writing tests.