Implement basic overload support via a new builtin, __builtin_overload.

__builtin_overload takes 2 or more arguments:
0) a non-zero constant-expr for the number of arguments the overloaded 
   functions will take
1) the arguments to pass to the matching overloaded function
2) a list of functions to match.

The return type of __builtin_overload is inferred from the function whose args
match the types of the arguments passed to the builtin.  For example:

float a;
float sinf(float);
int   sini(int);

float b = __builtin_overload(1, a, sini, sinf);

Says that we are overloading functions that take one argument, and trying to 
pass an argument of the same type as 'a'.  sini() does not match since it takes
and argument of type int.  sinf does match, so at codegen time this will turn
into float b = sinf(a);

llvm-svn: 46132