Remove some TextUnit->usize escapees

[CAD97]

As spotted during a review of all uses of text_unit::TextUnit::to_usize. Legitimate uses do remain.

[CAD97]

There seem to have been some spurious heavy test failures on 25aa385 (which I force pushed over because it was misformatted): https://github.com/rust-analyzer/rust-analyzer/runs/504765598?check_suite_focus=true

    Running target/debug/deps/heavy_tests-1d886a305502eda0

running 9 tests
thread 'test server' panicked at 'all snapshots should be dead: RwLock { data: Vfs { n_roots: 11, n_files: 611, n_pending_changes: 0 } }', crates/rust-analyzer/src/main_loop.rs:260:15
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
test diagnostics_dont_block_typing ... FAILED
test preserves_dos_line_endings ... ok
test test_format_document ... ok
test test_format_document_2018 ... ok
test test_missing_module_code_action ... ok
test test_missing_module_code_action_in_json_project ... ok
[ERROR rust_analyzer::main_loop] loading workspace failed: Failed to find Cargo.toml for path /tmp/.tmpSSoPil
    
    Caused by:
        can't find Cargo.toml at /tmp/.tmpSSoPil, due to no Cargo.toml file found
test test_runnables_no_project ... ok
test test_runnables_project ... ok
test completes_items_from_standard_library ... test completes_items_from_standard_library has been running for over 60 seconds
error: test failed, to rerun pass '-p rust-analyzer --test heavy_tests'
test completes_items_from_standard_library ... ok

failures:

---- diagnostics_dont_block_typing stdout ----
thread 'diagnostics_dont_block_typing' panicked at 'called `Result::unwrap()` on an `Err` value: Any', /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/jod-thread-0.1.0/src/lib.rs:13:17


failures:
    diagnostics_dont_block_typing

test result: FAILED. 8 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

[matklad]

bors r+

[bors]

Build succeeded

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