i could easily google it but, what's LLVM? (me googling it would not translate automatically into me writing up an answer for everybody, however, LLVM seems to be the name of a project that includes clang)
Other projects are included in LLVM. Clang is the C/C++ compiler. There is a C standard library, an OpenCL standard library, a C++ standard library (all inventively named lib<language>). There is also a library to support compiler builtins called compiler-rt, and a library to support OpenMP called openmp, and an implementation of C++ parallel executors in pstl. There is also a linker (lld) and a debugger (lldb). There is also a machine code optimizer called bolt, and yet another compiler framework called MLIR. Finally, there are several different Fortran compilers all called flang (don't ask).
Llvm is everywhere. There’s a good chance llvm compiled some or all of the software you’re using to read this comment.
I mean, that's not the reason Rust is as fast as C. It's because Rust semantically doesn't include mandatory slower things.
The compiler it uses is an implementation detail.