[libspirv] Clone remangled functions on name clash (private default AS)

[wenju-he]

When the target’s default address space is private (e.g., SPIR), simultaneous pointer address space remangling and long -> long long remangling can cause clone name collisions, preventing the remangled function from being cloned.

Example:
_Z1fPm -> remangled to _Z1fPU3AS0y; remangler clones back to _Z1fPm to preserve the original.
Later, _Z1fPU3AS4m -> remangled to _Z1fPy; we need to clone _Z1fPy to _Z1fPm, but _Z1fPm already exists.

Fix:
Append a temporary suffix to avoid clashes (e.g., _Z1fPm$TmpSuffix). Remove the suffix in post-processing, replacing the old _Z1fPm (clone of _Z1fPU3AS0y) with the new remangled implementation.

llvm-diff shows no change for targets whose default AS isn't private: remangled-l64-signed_char.libspirv-nvptx64-nvidia-cuda.bc and remangled-l64-unsigned_char.libspirv-amdgcn-amd-amdhsa.bc.

[wenju-he]

run_prebuilt_e2e_tests (Intel Battlemage Graphics with Level Zero, [Windows,bmg]) _ Intel Battlemage Graphics with Level Zero fails are unrelated because libspirv isn't used in Level Zero testing. I have run the task 3 times:
1st fail: Test d:\github_work\llvm\llvm\build-e2e\newoffloaddriver\output\buffer.cpp.tmp1.out hung!
2nd fail: Test d:\github_work\llvm\llvm\build-e2e\freefunctioncommands\output\launch_reduce.cpp.tmp.out hung!
3rd fail: Test d:\github_work\llvm\llvm\build-e2e\freefunctioncommands\output\launch_reduce.cpp.tmp.out hung!

@intel/llvm-gatekeepers please merge, thanks.