Tracking issue for the LLVM 21 upgrade

[alexrp]

• [NVPTX] Add tcgen05 alloc/dealloc intrinsics llvm/llvm-project#124961 (data layout change)
• [NVPTX] Add support for Shared Cluster Memory address space [1/2] llvm/llvm-project#135444 (data layout and address space changes)
• f2f36c4
• [SystemZ] Add support for half (fp16) llvm/llvm-project#109164
• [NVPTX] Basic support for fp128 as a storage type llvm/llvm-project#136006
• CPU features are not passed to clang when assembling #10411
• On wasm fmt print of u64 packed struct triggers WebAssembly translation error  #20966
• [AMDGPU] Set AS8 address width to 48 bits llvm/llvm-project#139419 (data layout change)
• [Clang] Partially fix m68k alignments llvm/llvm-project#144740 (C alignment change)
• [LoopSimplifyCFG] Add check for missing loop preheader llvm/llvm-project#149743
• [clang][LoongArch] Add support for the _Float16 type llvm/llvm-project#141703
• [Driver] Default to -mcpu=ultrasparc3 on Solaris/SPARC llvm/llvm-project#149990
• Adding Loongarch64 to OpenBSD parts llvm/llvm-project#149737
• C backend should use __attribute__((nonstring)) when applicable #24545

Previous upgrade: #22014