Compatibility, PE32+, and function labels

[laomaiweng]

This PR brings several changes that kind of build upon each other, but that I can likely split upon request. :)

In commit order, the changes are:

• compatibility fix with latest LLVM master (actually llvm/llvm-project@b7402edce315, there were more recent changes to PDB-related code but I don't think they affect pdbgen)
• compatibility with IDA 7.4+
• compatibility with Python 3, since IDA 7.4+ is moving towards Python 3 (but the changes should not break Python 2 compatibility)
• PE32+ support (basically it turns a bunch of uint32_t into uint64_t and things seem to just work)
• support for rebased IDBs by exporting RVAs instead of EAs to JSON
• optional export of function labels in the generated PDB

As a side note with regards to LLVM compatibility, in the future wouldn't it be easier to settle on a tagged LLVM release instead of always chasing after master's breaking changes?

Feel free to suggest changes to the PR as you think is needed. And by the way, thanks a lot for this awesome tool!

[laomaiweng]

(The AppVeyor build fails because of the 1-hour time limit, which LLVM exceeds.)

[Mixaill]

Thanks for your contribution!