[flang] Fix implicit declarations in statement functions

If a symbol (that is not a dummy argument) is implicitly declared inside
a statement function, don't create it in the statement function's scope.
Instead, treat statement functions like blocks when finding the inclusive
scope and create the symbol there.

Add a new flag, StmtFunction, to symbols that represent statement functions.

Differential Revision: https://reviews.llvm.org/D84588