[CIR][CodeGen] Use MapVector instead of StringMap for Replacements

[xaerru]

Fix #1938

Minimized benchmark:

#include <vector>

class my_class {
public:
    std::vector<void*> my_vec;
};

my_class inst;

When using llvm::StringMap transitive replacements may be out of order. So use llvm::MapVector which is also used in clang's CodeGen.

clangir/clang/lib/CodeGen/CodeGenModule.h

Line 439 in 0c130cd

llvm::MapVector<StringRef, llvm::TrackingVH<llvm::Constant>> Replacements;

I couldn't reproduce the MRE with #include "std-cxx.h", any other way to add the test?

[xaerru]

Ping.

[bcardosolopes]

Hi @xaerru, can you send this to upstream? https://discourse.llvm.org/t/clangir-transition-plans-for-the-clangir-incubator/89583/2, I'm gonna accept+merge this given it's a bug fix, but it'd be great it that fixed upstream too.

[bcardosolopes]

LGTM