[flang] Checks for pointers to intrinsic functions

Check that when a procedure pointer is initialised or assigned with an intrinsic
function, or when its interface is being defined by one, that intrinsic function
is unrestricted specific (listed in Table 16.2 of F'2018).

Mark intrinsics LGE, LGT, LLE, and LLT as restricted specific. Getting their
classifications right helps in designing the tests.

Differential Revision: https://reviews.llvm.org/D112381