cg_llvm: Remove the `mod llvm_` hack, which should no longer be necessary
This re-export was introduced in c76fc3d804, as a workaround for #53912.
In short, there was/is an assumption in some LLVM LTO code that symbol names would not contain `.llvm.`, but legacy symbol mangling would naturally produce that sequence for symbols in a module named `llvm`.
This was later “fixed” by adding a special case to the legacy symbol mangler in #61195, which detects the sequence `llvm` and emits the `m` in an escaped form. As a result, there should no longer be any need to avoid the module name `llvm` in the compiler itself.
(Symbol mangling v0 avoids this problem by not using `.` in the first place, outside of the “vendor-specific suffix”.)
c549268a4a