[flang] Accept ENTRY names in generic interfaces

ENTRY statement names in module subprograms were not acceptable for
use as a "module procedure" in a generic interface, but should be.
ENTRY statements need to have symbols with place-holding
SubprogramNameDetails created for them in order to be visible in
generic interfaces.  Those symbols are created from the "program
tree" data structure.  This patch adds ENTRY statement names to the
program tree data structure and uses them to generate SubprogramNameDetails
symbols.

Differential Revision: https://reviews.llvm.org/D117345