PostfixExpression: move DWARF generator out of NativePDB internals

Summary:
The new dwarf generator is pretty much a verbatim copy of the one in
PDB.

In order to write a pdb-independent test for it, I needed to write a
dummy "symbol resolver", which (together with the fact that I'll need
one more for breakpad-specific resolution logic) prompted me to create a
more simple interface for algorithms which replace or "resolve"
SymbolNodes. The resolving algorithms in NativePDB have been updated to
make use of that too.

I have removed a couple of NativePDB tests which weren't testing
anything pdb-specific and where the tested functionality was covered by
the new format-agnostic tests I have added.

Reviewers: amccarth, clayborg, aleksandr.urakov

Subscribers: aprantl, markmentovai, lldb-commits, jasonmolenda, JDevlieghere

Differential Revision: https://reviews.llvm.org/D61056

llvm-svn: 359288