Add importing cxx function decls and the basic requisite AbstractionPattern support.

[pschuh]

Known issues so far:

• Type-checking doesn't seem to know the c++ rules for selecting const
  vs non-const.
• The convention for passing arguments doesn't appear correct yet.
• Objective-c++ details are not handled yet.
• Methods on base classes and virtual functions will not be found.
• Const methods cannot be marked as non-mutating. This is because the
  fact that self is indirect is getting lost in the lowering process (becomes @in)

This is just some ImportDecl.cpp changes and CXXMethodConventions + the boilerplate in AbstractionPattern. There will probably need to be more to fully support methods.

[rjmccall]

Just a bunch of minor comments.

[jrose-apple]

This seems pretty straightforward, and I admit I can't comment too much on the AbstractionPattern parts. However, two things stuck out to me:

• Where do we treat static C++ methods differently from non-static ones? Or should we treat static ones like C functions, and the AbstractionPattern for C++ methods is really "C++ instance methods"?

• Is it possible for us to avoid adding all the new kinds of AbstractionPattern? A CXXMethodDecl is already a kind of clang::FunctionDecl, and we can already import regular functions as methods using NS_SWIFT_NAME. On the Swift side, a C++ instance method just has a different set of calling conventions but otherwise seems the same.

I admit this second part is born out of people asking about other importers, even if that's way more in the hypothetical future. I'm not going to go as far as to say everything needs to fit into the existing mold, or that you have to generalize everything so that we can implement a Fortran importer, but if we can figure out what needs to be generalized as part of your work, or at least avoid special-casing a ton of things, that's good.

I left more comments on the ClangImporter part, small as it was, but I don't think any of them are too significant.

[rjmccall]

Minor tweak, but looking really good.

[jrose-apple]

I think I have no remaining comments; leaving it to @rjmccall for final approval.

[pschuh]

@swift-ci please test

[pschuh]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 3037d59196c3befebf7dfa6210061752d491b26d

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 3037d59196c3befebf7dfa6210061752d491b26d

[pschuh]

@rjmccall John, could you take a quick look at this? It should be straightforward to approve.

[rjmccall]

I was just on vacation for three days; looking at it now.

[rjmccall]

LGTM