[flang] Perform definability checks on LHS of assignment

If the pure context check succeeds, call `WhyNotModifiable` to verify
the LHS can be modified.

Detect assignment to whole assumed-size array.

Change `IsVariable` to return false for a parameter or a component or
array reference whose base it a parameter.

When analyzing an assignment statement, report an error if the LHS is
a constant expression. Otherwise it might get folded and when we detect
the problem later the error will be confusing.

Handle Substring on LHS of assignment. Change ExtractDataRef and IsVariable
to work on a Substring.

Fix IsImpliedShape and IsAssumedSize predicates in ArraySpec.

Fix C709 check in check-declarations.cpp.

Original-commit: flang-compiler/f18@f2d2657aab051b938ec4e4702926aadb436c19aa
Reviewed-on: https://github.com/flang-compiler/f18/pull/1050