In mkSynBinding, move any attribute written with the explicit 'return:' target
from the binding's prefix attributes into SynValData.SynValInfo.retInfo.
This makes the syntactic placement (which the parser puts on the binding)
match the semantic intent (the attribute targets the method's return value).

Fixes:
  - dotnet#19020: [<return: X>] silently dropped on class members
  - dotnet#17904: false-positive AllowMultiple=false when [<X>] and [<return: X>]
    appear on the same member

…-pattern detection

After SynInfo.RotateReturnAttributes moves [<return: X>] from the binding's
prefix attributes into SynValData.SynValInfo.retInfo, two downstream consumers
also need updating:

- TcNormalizedBinding's retAttribs computation now type-checks attrs already
  in valSynData's return SynArgInfo, so isStructRetTy/argAndRetAttribs work
  for [<return: Struct>] on partial active patterns.
- ActivePatternElemsOfValRef now classifies the flag bag from
  ValReprInfo's result ArgReprInfo rather than scanning vref.Attribs.

Fixes recursive struct active patterns (e.g. let rec (|HasOne|_|)).

Drop the module-let case from the dotnet#19020 test (module bindings were
never affected). Rename tests with 'Issue NNNNN -' prefix and tighten
source samples and failure messages.

Verifies that [<CompilationRepresentation(Instance)>] on a union-type
member is not rotated to the return value.

…aration

Three tests verify the FCS Symbols API correctly surfaces:
- [<X>] on FSharpMemberOrFunctionOrValue.Attributes only
- [<return: X>] on ReturnParameter.Attributes only
- Both, independently, when both targets are used on the same member

Uses the marker-based Checker.getSymbolUse API and the existing
HasAttribute<'T>() member against System.ComponentModel.DescriptionAttribute
(AttributeTargets.All) to avoid new test-local helpers.