Use [`direct_headers`](https://bazel.build/rules/lib/CompilationContext#direct_headers)
from the Bazel/Blaze compilation context instead of _all_ transitive headers.
For the clang based generator, this means we don't try to parse
`textual_headers`, which will fail (they are by definition not
stand-alone, after all).
PiperOrigin-RevId: 431899423
Change-Id: I7a9dfa0dd93eba14b506b0e7ca6db3ed59b55dd6
60fcc5b63e