DWARFIndex: simplify GetFunctions methods

Now that Apple index determines method-ness straight from the debug
info, we don't need to resolve the functions into SymbolContexts inside
the Index classes. This removes the need for callback arguments and
allows us to pull the common parts out of the two implementations of
these functions back into the SymbolFileDWARF class.

Reviewers: JDevlieghere, clayborg

Subscribers: aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D47147

llvm-svn: 334004