Swift: Exclude SDK include paths

Populate `CMAKE_Swift_IMPLICIT_INCLUDE_DIRECTORIES` with the macOS SDK's
include directory so that we filter such implicit directories out of
Swift targets.

Fixes: #19845

Tests/RunCMake/XcodeProject/use_cmath.cpp

@@ -0,0 +1,6 @@
+#include <cmath>
+
+bool foo(double arg)
+{
+  return std::isfinite(arg);
+}