[flang] Make 'num_images()' intrinsic

[flang] Make 'num_images()' intrinsic

I added 'num_images()' to the list of functions that are evaluated as intrinsic. I also added a test file in flang/test/Semantics to test calls to 'num_images()'. There was a call to 'num_images()' in flang/test/Semantics/call10.f90 that expected an error, now it no longer produces an error. So I edited that file accordingly. I also edited the intrinsics unit test to add further testing of 'num_images()'.

Differential Revision: https://reviews.llvm.org/D83142