[llvm] [ThinLTO][NFC] Refactor FileCache (PR #110463)

[Ellis Hoag via llvm-commits]

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@@ -54,9 +54,29 @@ using AddStreamFn = std::function<Expected<std::unique_ptr<CachedFileStream>>(
 ///
 /// if (AddStreamFn AddStream = Cache(Task, Key, ModuleName))
 ///   ProduceContent(AddStream);
-using FileCache = std::function<Expected<AddStreamFn>(
+using FileCacheFunction = std::function<Expected<AddStreamFn>(
     unsigned Task, StringRef Key, const Twine &ModuleName)>;
 
+struct FileCache {
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ellishg wrote:

Should we move the docs for `FileCacheFunction` to `FileCache` since I think that will be the primary struct we should use. Also, we should add brief docs for `FileCacheFunction` to explain the arguments.

https://github.com/llvm/llvm-project/pull/110463