[LLVM] Fixes up to the latest LLVM21

[cbalint13]

This PR fix TVM use with the latest LLVM version 21.

• At this time LLVM21 is available as a release candidate.
• Double checks for backward compatibility down to LLVM10.

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Notes (LLVM21):

• Explicit conversion of llvm::Triple <-> str() is a requirement, it is not deduced anymore.
• As of llvm#140615 , intrinsic llvm.nvvm.barrier0 is now llvm.nvvm.barrier.cta.sync.aligned.all(0).
• ORCJIT's linker requires a llvm::Expected protection from now.

Backward version checks:

$ rpm -q gcc-c++
gcc-c++-15.1.1-5.fc43.1.x86_64

for llvm = {10,11,12,13,14,15,16,17,18,19,20,21} do make; make cpptest; make test