[flang][openacc] Fix function name resolution in acc routine

When acc routine is in a function, the first symbol resolved
was the function result and not the function name itself. It was
then failing the deferred attachment because the mangled name
had the entity attach to it. This patch fix the name resolution
for the function name in acc routine directive.

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D158868