[llvm] [GC] Use `MapVector` for `GCStrategyMap` (PR #132729)

[via llvm-commits]

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@@ -151,9 +152,9 @@ class GCFunctionInfo {
   size_t live_size(const iterator &p) const { return roots_size(); }
 };
 
-struct GCStrategyMap {
-  StringMap<std::unique_ptr<GCStrategy>> StrategyMap;
-
+class GCStrategyMap
+    : public MapVector<std::string, std::unique_ptr<GCStrategy>> {
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paperchalice wrote:

May be a bad practice here.

https://github.com/llvm/llvm-project/pull/132729