[lldb] fix stepping through POSIX trampolines

The DynamicLoaderPOSIXDYLD::GetStepThroughTrampolinePlan() function was
doing the symbol lookup using the demangled name. This stopped working
with https://reviews.llvm.org/D118814. To get things working again, just
use the mangled name for the lookup instead.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D127999