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.
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