[flang] Prevent lowering crash by properly updating symbol pointer

Kind of an edge case.  When a MODULE FUNCTION or SUBROUTINE
interface is defined by a MODULE PROCEDURE in the same program
unit, ensure that the symbol table pointer in the parse tree is
updated to point to the SubprogramDetails symbol for the
interface, and not left pointing to what should soon become
a dead SubprogramNameDetails symbol.

Differential Revision: https://reviews.llvm.org/D154380