Rename `src/bootstrap/native.rs` to `llvm.rs`

[jyn514]

The comment at the top of the file says

rust/src/bootstrap/native.rs

Line 1 in ae5473c

//! Compilation of native dependencies like LLVM.

so presumably the logic is that "native" includes C/C++ code that isn't LLVM. But the only C code that's not LLVM is

rust/src/bootstrap/native.rs

Line 899 in ae5473c

pub struct TestHelpers {

I think we should move TestHelpers to test.rs and rename native.rs to llvm.rs to make it easier to find for new contributors.

cc @compiler-errors, I think you asked about where to find this code recently.

[jkvargas]

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[rohaquinlop]

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[jkvargas]

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