On most Unix-like systems, wide character literal is 32-bit long
and encodes a Unicode code point. On Windows, that is 16-bit
long and encodes a UTF-16 code unit. Clearly, there's a portability
issue here. Personally I've never used wide characters in my code
as I didn't find it useful.

Being said that, some header files contain wide character literal,
so we need to support that so that chibicc can include such files.

We assume that source files are always encoded in UTF-8.

For now, L'' is equivalent to ''.

_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

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;