[llvm] [RuntimeLibcalls] Rework LibcallImplCallingConvs to reduce stack usage (PR #174689)

[Matt Arsenault via llvm-commits]

================

----------------
arsenm wrote:

Maybe preserve the static size:
```
  std::unique_ptr<std::array<CallingConv::ID, RTLIB::NumLibcallImpls>>
      LibcallImplCallingConvs =
          std::make_unique<std::array<CallingConv::ID, RTLIB::NumLibcallImpls>>();
```

https://github.com/llvm/llvm-project/pull/174689