[flang] Move check for statement function in BLOCK construct

A BLOCK construct may not contain a statement function definition;
but it may of course contain an assignment statement with an array
element on its left-hand side that looks like a statement function
definition.  These misparsed statement functions are converted
into assignment statements during semantics once it is clear what
they are.  Move the C1107 check for a statement function definition
in a block construct into declaration checking, which is where it
probably should have been in the first place anyway.

Differential Revision: https://reviews.llvm.org/D145112