New _slang_adapt_call() function.

New _slang_adapt_call() function.

This is used to modify function calls (when possible) to make the arguments
map to the function parameters.  This includes "unrolling" vector types
and doing casts.

Example:
   vec2 v2 = vec2(1.2, 3.4)
   ivec3 iv = ivec3(false, v2);
Is converted into:
   ivec3 iv = ivec3(int(false), int(v2[0]), int(v2[1]))