scons/windows: Support build with LLVM 11.

Added LLVMFrontendOpenMP.
Removed LLVMX86Utils.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Part-of: <!7242>
36 jobs for !7242 with mingw-llvm-11 in 3 minutes and 53 seconds (queued for 3 seconds)
merge request