[clang][CodeGen] Fix in codegen for __builtin_popcountg/ctzg/clzg (#90845)

Make sure that the result from the popcnt/ctlz/cttz intrinsics is
unsigned casted to int, rather than casted as a signed value, when
expanding the __builtin_popcountg/__builtin_ctzg/__builtin_clzg
builtins.

An example would be
  unsigned _BitInt(1) x = ...;
  int y = __builtin_popcountg(x);
which previously was incorrectly expanded to
  %1 = call i1 @llvm.ctpop.i1(i1 %0)
  %cast = sext i1 %1 to i32

Since the input type is generic for those "g" versions of the builtins
the intrinsic call may return a value for which the sign bit is set
(that could typically for BitInt of size 1 and 2). So we need to emit a
zext rather than a sext to avoid negative results.