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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Rough testing found that this change reduced the stack memory usage of module analysis registration of RuntimeLibraryAnalysis by 22544 bytes (RelWithDbgInfo GCC build on Linux).

Can we just change that to do not do a full value copy instead? This should just live in one non-stack place as a constant 



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