Given some C/C++ file foo.c:
> clang -S -emit-llvm foo.c
Produces foo.ll which is an LLVM IR file.
The -emit-llvm option can also be passed to the compiler front-end directly, and not the driver by means of -cc1:
> clang -cc1 foo.c -emit-llvm
Produces foo.ll with the IR. -cc1 adds some cool options like -ast-print. Check out -cc1 --help for more details.
To compile LLVM IR further to assembly, use the llc tool:
> llc foo.ll
Produces foo.s with assembly (defaulting to the machine architecture you run it on). llc is one of the LLVM tools – here is its documentation.