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.

src/librustc_codegen_llvm/llvm/ffi.rs

@@ -588,7 +588,6 @@ pub mod debuginfo {
             const FlagIntroducedVirtual   = (1 << 18);
             const FlagBitField            = (1 << 19);
             const FlagNoReturn            = (1 << 20);
-            const FlagMainSubprogram      = (1 << 21);
         }
     }
 
@@ -603,6 +602,7 @@ pub mod debuginfo {
             const SPFlagLocalToUnit       = (1 << 2);
             const SPFlagDefinition        = (1 << 3);
             const SPFlagOptimized         = (1 << 4);
+            const SPFlagMainSubprogram    = (1 << 5);
         }
     }