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