b21c438eb5
cg_llvm: Stop using `as_c_char_ptr` for coverage-related bindings
[As explained by a note in `ffi.rs`](8a1b39995e/compiler/rustc_codegen_llvm/src/llvm/ffi.rs (L4-L11)), passing strings and byte slices through FFI is more convenient if we take advantage of the fact that `*const c_uchar` and `*const c_char` have the same ABI.
Doing so avoids having to rely on a special helper function, since we can just call `as_ptr` instead.
(The same logic applies to every other binding that currently uses the `as_c_char_ptr` helper; I just haven't adjusted all of them yet.)
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As a drive-by change, this PR also marks some coverage-related FFI bindings as `safe`.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.