Support "presence" mode in coverage instrumentation

[woodruffw]

MCP: rust-lang/compiler-team#1002

Tracker: #158408

This adds a new unstable variant of -C instrument-coverage, namely -C instrument-coverage=presence. "Presence" instrumentation is what LLVM calls "single-byte counter" coverage, although the latter is arguably misleading because it's really a 1-bit coverage mode that just happens to use bytes as the coverage map representation.

Separately, this PR adds a new alias, -C instrument-coverage=counter, which is equivalent to the current bare form of -C instrument-coverage (or -C instrument-coverage=yes).

Notes/TODOs:

• Move the documentation changes out of this PR and into rustc-dev-guide.

[rustbot]

Some changes occurred in coverage instrumentation.

cc @Zalathar

Some changes occurred in coverage tests.

cc @Zalathar

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustc-dev-guide is developed in its own repository. If possible, consider making this change to rust-lang/rustc-dev-guide instead.

cc @BoxyUwU, @tshepang

Some changes occurred in coverage instrumentation.

cc @Zalathar

Some changes occurred in coverage instrumentation.

cc @Zalathar

[rustbot]

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[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

  PR_CI_JOB: 1
  IMAGE: pr-check-2
##[endgroup]
    Updating crates.io index
error: failed to get `icu_normalizer` as a dependency of package `idna_adapter v1.2.0`
    ... which satisfies dependency `idna_adapter = "^1"` of package `idna v1.0.3`
    ... which satisfies dependency `idna = "^1.0"` of package `cookie_store v0.21.1`
    ... which satisfies dependency `cookie_store = "^0.21.1"` of package `ureq v3.0.8`
    ... which satisfies dependency `ureq = "^3"` of package `citool v0.1.0 (/home/runner/work/rust/rust/src/ci/citool)`

Caused by:
  failed to load source for dependency `icu_normalizer`

Caused by:
  unable to update registry `crates-io`

Caused by:
  download of ic/u_/icu_normalizer failed

Caused by:
  curl failed

Caused by:

[woodruffw]

CI failure here appears to be the known HTTP/2 flake fixed with rust-lang/cargo#16903

[rust-bors]

☔ The latest upstream changes (presumably #158593) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.