Rollup merge of rust-lang#125345 - durin42:thin-link-bitcode, r=bjorn3

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/driver/aot.rs

@@ -200,7 +200,7 @@ fn produce_final_output_artifacts(
     // to get rid of it.
     for output_type in crate_output.outputs.keys() {
         match *output_type {
-            OutputType::Bitcode => {
+            OutputType::Bitcode | OutputType::ThinLinkBitcode => {
                 // Cranelift doesn't have bitcode
                 // user_wants_bitcode = true;
                 // // Copy to .bc, but always keep the .0.bc. There is a later