[flang] Refine handling of SELECT TYPE associations in analyses (#128935)

A few bits of semantic checking need a variant of the
ResolveAssociations utility function that stops when hitting a construct
entity for a type or class guard. This is necessary for cases like the
bug below where the analysis is concerned with the type of the name in
context, rather than its shape or storage or whatever. So add a flag to
ResolveAssociations and GetAssociationRoot to make this happen, and use
it at the appropriate call sites.

Fixes https://github.com/llvm/llvm-project/issues/128608.

flang/lib/Semantics/expression.cpp

@@ -3289,7 +3289,7 @@ const Assignment *ExpressionAnalyzer::Analyze(const parser::AssignmentStmt &x) {
             dyType && dyType->IsPolymorphic()) { // 10.2.1.2p1(1)
           const Symbol *lastWhole0{UnwrapWholeSymbolOrComponentDataRef(lhs)};
           const Symbol *lastWhole{
-              lastWhole0 ? &lastWhole0->GetUltimate() : nullptr};
+              lastWhole0 ? &ResolveAssociations(*lastWhole0) : nullptr};
           if (!lastWhole || !IsAllocatable(*lastWhole)) {
             Say("Left-hand side of assignment may not be polymorphic unless assignment is to an entire allocatable"_err_en_US);
           } else if (evaluate::IsCoarray(*lastWhole)) {