ac: reset LLVM command line parser

ac: reset LLVM command line parser

In some scenario ac_init_llvm_target may be called twice,
but the LLVM library won't have been unloaded between
the 2 calls, leading to a LLVM warning being printed.

Example pseudo-code to trigger this for radeonsi:
   gbm_create_device();
   eglInitialize();
   eglTerminate();
   gbm_device_destroy();
   gbm_create_device();
   eglInitialize();
   eglTerminate();
   gbm_device_destroy();

To avoid the warning message from LLVM, clear the command line
parser state before calling LLVMParseCommandLineOptions.

This might fix https://gitlab.freedesktop.org/mesa/mesa/-/issues/5960

This is done only on LLVM 12+ because it seems to break some apps
on LLVM 11 (there has been some work post LLVM 11 release to refactor
CommandLine.cpp, see 42f588f39c5c and the following commits).

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16587>