[flang] Legacy extension intrinsic functions IMAG, IZEXT, JZEXT

Support these legacy extension intrinsic functions with unambiguous
semantics in those existing compilers that support them by means
of recognizing them as aliases for standard intrinsics (IMAG) or
with simple rewrites (IZEXT, JZEXT).  Note that ZEXT has different
semantics in different existing compilers, so we will not support it
due to lack of a broad unambiguous precedent.

Differential Revision: https://reviews.llvm.org/D132154