[mlir] fix DiagnosedSilenceableFailure::takeDiagnostics

This function was returning an rvalue reference to an object that was
also cleared via RAII when the function returned, making it always
return an empty object. Make it accept the mutable reference to the
object instead to avoid this dangerous behavior.

Reviewed By: guraypp

Differential Revision: https://reviews.llvm.org/D134948