Add `parse_headers` support to the default Unix toolchain

[fmeum]

RELNOTES: The default Unix C++ toolchain now supports the parse_headers feature to validate header files with --process_headers_in_dependencies.

[fmeum]

@bazel-io fork 7.1.0

[fmeum]

@iancha1992 This is not a hard blocker for the 7.1.0 release, but getting it in there would help maintaining C++ modules in the BCR (cc @meteorcloudy).

[iancha1992]

@fmeum do you still want to merge this?

[fmeum]

@iancha1992 Yes, it's just waiting for a review.

@comius Friendly ping :⁠-⁠)

[fmeum]

@pzembrod I unassigned Ivo and moved this over to you. Let me know if you have any questions!

[Wyverald]

@pzembrod Is this still on track for 7.2.0? We're aiming for RC1 next Monday.

[iancha1992]

The changes in this PR have been included in Bazel 7.2.0 RC1. Please test out the release candidate and report any issues as soon as possible. If you're using Bazelisk, you can point to the latest RC by setting USE_BAZEL_VERSION=7.2.0rc1. Thanks!