PostfixExpression: move parser out of NativePDB internals

Summary:
The postfix expressions in PDB and breakpad symbol files are similar
enough that they can be parsed by the same parser. This patch
generalizes the parser in the NativePDB plugin and moves it into the
PostfixExpression file created in the previous commit (r358976).

The generalization consists of treating any unrecognised token as a
"symbol" node (previously these would only be created for tokens
starting with "$", and other token would abort the parse). This is
needed because breakpad symbols can also contain ".cfa" tokens, which
refer to the frame's CFA.

The cosmetic changes include:
- using a factory function instead of a class for creating nodes (this
  is more generic as it allows the same BumpPtrAllocator to be used for
  other things too)
- using dedicated function for parsing operator tokens instead of a
  DenseMap (more efficient as we don't need to create the DenseMap every
  time).

Reviewers: amccarth, clayborg, JDevlieghere, aleksandr.urakov

Subscribers: jasonmolenda, lldb-commits, markmentovai, mgorny

Differential Revision: https://reviews.llvm.org/D61003

llvm-svn: 359073