Rollup merge of #119034 - Zalathar:ignore-mode, r=davidtwco

Allow coverage tests to ignore test modes, and to enable color in coverage reports

This PR adds two new header directives to compiletest, intended for use by coverage tests (and by #119033 in particular).

The new headers are:

- `// ignore-mode-{mode}` causes a test to not be run in a particular compiletest mode (e.g. `ignore-mode-coverage-run`).
  - This can theoretically be used by any test, but coverage tests are currently the only ones that automatically run in multiple modes, so it's not very useful for other kinds of test.
- `// llvm-cov-flags: --use-color` makes `coverage-run` tests pass the flag `--use-color` when generating coverage reports.
  - For most tests, non-coloured reports are easier to read and more portable across platforms. But for #119033 specifically, we want to test that `llvm-cov` slices up source text correctly, which only happens when colour output is enabled.

Author: compiler-errors