Prevent compile parts of rustc when using cargo dev ra-setup

[dima74]

Currently after running cargo dev ra-setup the following lines are added to Cargo.toml:

[target]
rustc_data_structures = { path = ".../rust/src/librustc_data_structures" }
rustc_driver = { path = ".../rust/src/librustc_driver" }
rustc_errors = { path = ".../rust/src/librustc_errors" }
rustc_interface = { path = ".../rust/src/librustc_interface" }
rustc_middle = { path = ".../rust/src/librustc_middle" }

This pull request adds dependencies for rustc crates under cfg(NOT_A_PLATFORM), thus preventing them from compiling together with clippy:

[target.'cfg(NOT_A_PLATFORM)'.dependencies]
rustc_data_structures = { path = ".../rust/src/librustc_data_structures" }
rustc_driver = { path = ".../rust/src/librustc_driver" }
rustc_errors = { path = ".../rust/src/librustc_errors" }
rustc_interface = { path = ".../rust/src/librustc_interface" }
rustc_middle = { path = ".../rust/src/librustc_middle" }

---

This approach was originally proposed for IntelliJ Rust, and looks like it works for rust-analyzer too.

changelog: none

[rust-highfive]

r? @Manishearth

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[Manishearth]

@bors r+

[bors]

📌 Commit 6af9693 has been approved by Manishearth

[bors]

⌛ Testing commit 6af9693 with merge 7228368...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 7228368 to master...