_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

Previously, chibicc's sizeof accepted only an expression, so you
couldn't write something like `sizeof(int)`. Now it accepts that.

In the following example, `x` is defined as an alias for `int`.

  typedef x;

Below is valid C code where the second `t` is a local variable
of type int having value 3.

  typedef int t;
  t t = 3;

chibicc can now read complex type declarations such as below.

  long x;
  long int x;
  int long x;
  short x;
  short int x;
  int short x;
  long long x;
  long long int x;

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.

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

No functional change