[flang] Accept structure constructor value for polymorphic component

[flang] Accept structure constructor value for polymorphic component

Semantic analysis was emitting a bogus error message when a structure
constructor contains a monomorphic value for a (limited) polymorphic
component of a derived type.  The type compatibility test was too
strict; this patch relaxes it a little to allow values that could
be assigned or passed to a variable or dummy argument with that type.
Also add some quotes to an error message that was sometimes confusing
without them, and remove a repeated space character from another.

Differential Revision: https://reviews.llvm.org/D119744