[flang] Ensure that structure constructors fold parameter references

[klausler]

Summary

Structure contructors for instances of parameterized derived types
must have their components' values folded in the context of the values
of the type parameters.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Evaluate/fold.cpp

     FoldingContext &context, StructureConstructor &&structure) {
   StructureConstructor ctor{structure.derivedTypeSpec()};
   bool isConstant{true};
+  auto restorer{context.WithPDTInstance(structure.derivedTypeSpec())};
   for (auto &&[symbol, value] : std::move(structure)) {
     auto expr{Fold(context, std::move(value.value()))};
     if (IsPointer(symbol)) {

flang/test/Semantics/structconst05.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+! Ensure that PDT instance structure constructors can be folded to constants
+module m1
+  type :: pdt(k)
+    integer, len :: k
+    character(len=k) :: x, y = "def"
+  end type
+  type(pdt(4)) :: v = pdt(4)("abc")
+end module