clover: Fix build with llvm-12.

clover: Fix build with llvm-12.

Fix build error after LLVM commit c495dfe0268b ("[clang][cli] NFC:
Decrease the scope of ParseLangArgs parameters").

../src/gallium/frontends/clover/llvm/invocation.cpp: In function ‘std::unique_ptr<clang::CompilerInstance> {anonymous}::create_compiler_instance(const clover::device&, const string&, const std::vector<std::__cxx11::basic_string<char> >&, std::string&)’:
../src/gallium/frontends/clover/llvm/invocation.cpp:252:55: error: cannot convert ‘clang::PreprocessorOptions’ to ‘std::vector<std::__cxx11::basic_string<char> >&’
  252 |                                 c->getPreprocessorOpts(),
      |                                 ~~~~~~~~~~~~~~~~~~~~~~^~
      |                                                       |
      |                                                       clang::PreprocessorOptions

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4114
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8543>