[bazel] Enable header processing for C++ builds.

[vonosmas]

This would only work for projects that explicitly enable "parse_headers" feature (or for builds that specify this on command-line) - right now there are none, so this change shouldn't affect most builds. When "parse_headers" is enabled though, it would catch problems of incorrect/missing includes in header-only cc_libraries.

See https://bazel.build/docs/bazel-and-cpp#toolchain-features on why this option is a best practice for C++ projects.

[keith]

seems reasonable but does this pass for the whole build now? i assume it doesn't in lldb at least because of circular dependencies that we're avoiding

[vonosmas]

Yes, bazel build @@llvm-project//... succeeds (re-running now to confirm), because, as noted in the message, the project needs to enable parse_headers for this flag to take effect. If I add it for lldb, then yes, the build fails, as likely there's quite a bit of cleanup. It almost works for libc, so I intend to enable "parse_headers" explicitly in libc/BUILD.bazel soon, but not in other sub-projects.

[keith]

Ah thanks I forgot this requires the feature too

[vonosmas]

Thank you for the review!

[vonosmas]

Seems to have landed fine:

WARNING: Build option --process_headers_in_dependencies has changed, discarding analysis cache (this can be expensive, see https://bazel.build/advanced/performance/iteration-speed).
--
  | INFO: Analyzed 7245 targets (206 packages loaded, 38394 targets configured).
  | INFO: Found 4548 targets and 2697 test targets...
  | INFO: Elapsed time: 14.828s, Critical Path: 2.29s
  | INFO: 243 processes: 34381 action cache hit, 13 internal, 230 processwrapper-sandbox.
  | INFO: Build completed successfully, 243 total actions
  |  
  | Executed 0 out of 2697 tests: 2697 tests pass.