[lldb] Fix dwo variant of TestLibCxxFunction

The test was failing due to a bug in SymbolFileDWARF::FindFunctions --
the function was searching the main dwarf unit for DW_TAG_subprograms,
but the main unit is empty in case of split dwarf.  The fix is simple --
search the non-skeleton unit instead.

This bug went unnoticed because this function is expensive, and so one
generally avoids calling it.