Preserve ExceptionDispatchInfo.SetCurrentStackTrace across throw

[SimonCropp]

Bug

The ExceptionDispatchInfo.SetCurrentStackTrace polyfill wrote the captured trace to Exception._stackTraceString. The runtime resets that field when the exception is thrown, so the captured trace was lost as soon as the exception was thrown or surfaced through an await, which is the whole point of the API (e.g. TaskCompletionSource.SetException(ExceptionDispatchInfo.SetCurrentStackTrace(ex))).

Fix

Store the trace the way the BCL does: in _remoteStackTraceString, followed by the "End of stack trace from previous location" marker. The runtime preserves that field across throws and prepends it to Exception.StackTrace. If the field cannot be found, the previous _stackTraceString behaviour is kept as a fallback.

Test

ExceptionDispatchInfoSetCurrentStackTrace_SurvivesThrow and _SurvivesAwait capture the trace in a named NoInlining method, throw / await the exception, and assert the method name is still in StackTrace. Both failed on net48 before the fix; they pass on net462 / net48 (polyfill) and net8.0 / net10.0 (BCL).