swr/rast: Fix WriteBitcodeToFile usage with llvm-7.0.

swr/rast: Fix WriteBitcodeToFile usage with llvm-7.0.

Fix build error after llvm-7.0svn r325155 ("Pass a reference to a module
to the bitcode writer.").

  CXX      rasterizer/jitter/libmesaswr_la-JitManager.lo
rasterizer/jitter/JitManager.cpp:548:30: error: reference to type 'const llvm::Module' could not bind to an lvalue of type 'const llvm::Module *'
    llvm::WriteBitcodeToFile(M, bitcodeStream);
                             ^

Suggested-by: George Kyriazis <george.kyriazis@intel.com>
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-By: George Kyriazis <george.kyriazis@intel.com>