rustllvm relies on the `LLVMRustStringWriteImpl` symbol existing, but
this symbol was previously defined in a *downstream* crate
(rustc_codegen_llvm, which depends on rustc_llvm.
While this somehow worked under the old 'separate bootstrap step for
codegen' scheme, it meant that rustc_llvm could not actually be built by
itself, since it relied linking to the downstream rustc_codegen_llvm
crate.
Now that librustc_codegen_llvm is just a normal crate, we actually try
to build a standalone rustc_llvm when we run tests. This commit moves
`LLVMRustStringWriteImpl` into rustc_llvm (technically the rustllvm
directory, which has its contents built by rustc_llvm). This ensures
that we can build each crate in the graph by itself, without requiring
that any downstream crates be linked in as well.
The deprecated `LLVM{Get,Set}ValueName` only work with NUL-terminated
strings, but the `2` variants use explicit lengths, which fits better
with Rust strings and slices. We now use these in new helper functions
`llvm::{get,set}_value_name` that convert to/from `&[u8]`.
