[lldb] Remove IRExecutionUnit::CollectFallbackNames

The work that IRExecutionUnit::CollectFallbackNames is basically the
work that `CPlusPlusLanguage::GetDemangledFunctionNameWithoutArguments`
does already. It's also (at time or writing) specific to C++, so it can
be folded into `IRExecutionUnit::CollectCandidateCPlusPlusNames`.

Differential Revision: https://reviews.llvm.org/D109928