[clang] [lld] [llvm] Support -fpass-plugin + -fthinlto-index together (PR #183525)

[Nuri Amari via cfe-commits]

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@@ -50,7 +51,12 @@ struct Config {
   TargetOptions Options;
   std::vector<std::string> MAttrs;
   std::vector<std::string> MllvmArgs;
-  std::vector<std::string> PassPlugins;
+  // LTO will register both lists of plugins, but
+  // if an LTO client has already loaded a set of plugins,
+  // they should register them via LoadedPassPlugins.
+  // This is currently used by distributed thin-lto.
+  std::vector<std::string> PassPluginFilenames;
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NuriAmari wrote:

It's possible but I'm not necessarily convinced it's an improvement. We'd have to duplicate the plugin loading and error handling code for each user. It is also somewhat annoying in that each user has to make sure the pointed to pass plugins live long enough for LTO to use them since, as written, LTO does not take ownership. For example, in `createConfig` in [‎lld/MachO/LTO.cpp](https://github.com/llvm/llvm-project/pull/183525/changes#diff-b543a820fe9691347c01b16c3f49520bd86a80a56f4075d9868aca045f9794d7) we'd have to copy the plugins to the heap somehow.

https://github.com/llvm/llvm-project/pull/183525