Initialize targets enabled by the build script via cfg(llvm_component = "...").
N.B., this function can’t be moved to rustc_codegen_llvm because of the cfgs.
Opaque type that allows C++ code to write bytes to a Rust-side buffer,
in conjunction with RawRustStringOstream. Use this as &RustString
(Rust) and RustStringRef (C++) in FFI signatures.