[flang] Analyze intrinsic and user-defined assignments

[flang] Analyze intrinsic and user-defined assignments

Change expression analysis to do assignment statements as it currently
does call statements. Check there for defined assignment and set
`typedAssignment` in the `AssignmentStmt` node to contain the analyzed
assignment, either intrinsic or user-defined.
When `var = expr` is implemented by subroutine `sub`, the analyzed
assignment contains a procedure reference to `sub(var, (expr))`.

Add `IsDefinedAssignment` to decide based on types and ranks of lhs
and rhs whether is can be a defined assignment. The result is
tri-state because when they are both the same derived type it can
be either intrinsic or defined. Use this where a similar decision
is made in `check-declarations.cc`.

Change "Procedure referenced in PURE subprogram" error message to
contain the name of the procedure. If the reference is from a defined
assignment that name won't appear on the highlighted source line.

Original-commit: flang-compiler/f18@5c87071210ec74e9395805dd547c107e7d3bf7b6
Reviewed-on: https://github.com/flang-compiler/f18/pull/841