[flang] Keep fully qualified !fir.heap type for fir.freemem op

Re-introduce a fully qualified type on teh fir.freemem operation.
Since this is the only operation where the prefix gets elided in fir, this
patch make it fully qualified so the dialect syntax feels more consistent.

Reviewed By: vdonaldson

Differential Revision: https://reviews.llvm.org/D122839