Print detailed vector type information on diagnostics.

We never aka vector types because our attributed syntax for it is less
comprehensible than the typedefs. This leaves the user in the dark when
the typedef isn't named that well.

Example:
  v2s v; v4f w;
  w = v;

The naming in this cases isn't even that bad, but the error we give is
useless without looking up the actual typedefs.
t.c:6:5: error: assigning to 'v4f' from incompatible type 'v2s'

Now:
t.c:6:5: error: assigning to 'v4f' (vector of 4 'float' values) from
    incompatible type 'v2s' (vector of 2 'int' values)

We do this for all diagnostics that print a vector type.