rustc: Start implementing compat with LLVM 9

This commit doesn't actually migrate to LLVM 9, but it brings our own
C++ bindings in line with LLVM 9 and able to compile against tip of
tree. The changes made were:

* The `MainSubprogram` flag for debuginfo moved between flag types.
* Iteration of archive members was tweaked slightly and we have to
  construct the two iterators before constructing the returned
  `RustArchiveIterator` value.
* The `getOrInsertFunction` binding now returns a wrapper which we use
  `getCallee()` on to get the value we're interested in.