gallium/clover: pass -no-opaque-pointers to Clang

Since LLVM commit 702d5de4 Clang opaque pointers are enabled by default and it is incompatible with current Mesa code, see #6342 (closed)

A better implementation may be doable following suggestions from there:
https://github.com/llvm/llvm-project/issues/54970#issuecomment-1102254254

I don't have enough knowledge of Mesa and LLVM to implement the suggestion.

@currojerez said he would ACK this workaround anyway.

The workaround works by passing -no-opaque-pointers to clang so it works as before even if LLVM is built with opaque pointers by default.

Note: see also !16002 (merged) for another fix for another Mesa/LLVM mismatch.