[flang] Make 'this_image()' an intrinsic function

Added 'this_image()' to the list of functions that are evaluated as intrinsic.
Added IsCoarray functions to determine if an expression is a coarray (corank > 1).

Added save attribute to coarray variables in test file, this_image.f90.

reviewers: klausler, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D108059