rustc_codegen_llvm: add support for writing summary bitcode

Typical uses of ThinLTO don't have any use for this as a standalone
file, but distributed ThinLTO uses this to make the linker phase more
efficient. With clang you'd do something like `clang -flto=thin
-fthin-link-bitcode=foo.indexing.o -c foo.c` and then get both foo.o
(full of bitcode) and foo.indexing.o (just the summary or index part of
the bitcode). That's then usable by a two-stage linking process that's
more friendly to distributed build systems like bazel, which is why I'm
working on this area.

I talked some to @teresajohnson about naming in this area, as things
seem to be a little confused between various blog posts and build
systems. "bitcode index" and "bitcode summary" tend to be a little too
ambiguous, and she tends to use "thin link bitcode" and "minimized
bitcode" (which matches the descriptions in LLVM). Since the clang
option is thin-link-bitcode, I went with that to try and not add a new
spelling in the world.

Per @dtolnay, you can work around the lack of this by using `lld
--thinlto-index-only` to do the indexing on regular .o files of
bitcode, but that is a bit wasteful on actions when we already have all
the information in rustc and could just write out the matching minimized
bitcode. I didn't test that at all in our infrastructure, because by the
time I learned that I already had this patch largely written.

src/lib.rs

@@ -335,6 +335,10 @@ impl ThinBufferMethods for ThinBuffer {
     fn data(&self) -> &[u8] {
         unimplemented!();
     }
+
+    fn thin_link_data(&self) -> &[u8] {
+        unimplemented!();
+    }
 }
 
 pub struct GccContext {