[llvm] [bazel] Enable header processing for C++ builds. (PR #138934)

[Alexey Samsonov via llvm-commits]

vonosmas wrote:

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.
```


https://github.com/llvm/llvm-project/pull/138934