fix: Apply lint cap to recompilable dependency crates

[zalanlevai]

This PR fixes lint errors triggered when compiling recompilable dependency crates (i.e., --crate-kind=mutable-dep-for-external-tests, which are used for Cargo integration tests to generate a mostly unmodified dependency (the package's lib target) to perform cross-crate analysis against. These recompilable dependency crates are mostly identical to the original crate, except for the insertion of some constant items in a generated mod that store necessary information to be able to transparently recompile the crate as a meta-mutant for a specific integration test.

error: missing documentation for a module
  |
note: the lint level is defined here
 --> tests/ui/harness/cap_lints_recompilable_dep_crate.rs:7:11
  |
7 | #![forbid(missing_docs)]
  |           ^^^^^^^^^^^^

error: missing documentation for a constant

The missing lint cap on the recompilable dependency crates was causing issues with warn lints polluting the output because of the generated items, and deny and forbid lints outright preventing compilation, for example with the missing_docs lint.

This change mirrors the lint cap already applied in the regular test output compilation code path.

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This PR also fixes an issue that was revealed by Windows CI with one of the new test cases introduced: certain generated string literals containing strings from the environment, such as file paths, were not escaped. The second commit fixes this issue by applying escaping to these strings before they are written out as string literals.