Deduplicate rustc_demangle in librustc_codegen_llvm

This commit removes the crates.io dependency of `rustc-demangle` from
`rustc_codegen_llvm`. This crate is actually already pulled in to part
of the `librustc_driver` build and with the upcoming pipelining
implementation in Cargo it causes build issues if `rustc-demangle` is
left to its own devices.

This is not currently required, but once pipelining is enabled for
rustc's own build it will be required to build correctly.

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