`hdrs` includes are passed as `-isystem` to clang, suppressing important warnings

By default, most include paths passed to clang when using toolchains_llvm are passed as `-isystem`, which suppresses many warnings. This wasted a lot of time for me the other day when I accidentally declared a function with a bool return type and no return value. This is easily caught by clang under normal circumstances as I pass `-Wall` and `-Werror`, but due to the way headers are included, the warning/error never surfaced and it took hours to find.

For this target:
```
cc_libray(
name = "foo",
srcs=glob(["foo/*.cpp"]),
hdrs=glob(["foo/*.hpp"]),
)
```

I would expect things like `-Wreturn-type` violations in .hpp files to fail the build, but the way clang is currently invoked they don't.

Trace how toolchains_llvm turns hdrs include paths into clang flags for the shown cc_library target. Reproduce the case with -Wall and -Werror and a header containing a missing return, then compare -isystem with ordinary include handling. Done means the warning is surfaced without suppressing warnings from external headers.

Written by the indexing model from the issue text.