[flang] Fix mod file generation of derived type initializers... (#70511)

... when the derived type used in the structure constructor(s) is from
another module and not use-associated into the current module.

This came up in a test with a derived type component default initializer
of "c_null_ptr", which is replaced with the expression
"__builtin_c_ptr(address=0_8)"; the derived type name "__builtin_c_ptr"
is not available in the current scope, and the module file would fail
semantic analysis when USE'd.

The best solution that I found was to extend module file generation to
detect this case and handle it by inserting the right USE association to
the ultimate derived type symbol, possibly with renaming to a
compiler-created name in the case of a conflict.

To implement this transformation, it was necessary to fix the utility
evaluate::CollectSymbols() to include the derived type symbol from a
structure constructor. This involved extending the expression traversal
framework to visit the derived type spec of a structure constructor.
Extending CollectSymbols() caused a lowering test to fail mysteriously,
so I tracked down the code in PFTBuilder that didn't expect to see a
DerivedTypeDetails symbol and dealt with it there.