[clang][deps] During scanning don't emit warnings-as-errors that are ignored...

[clang][deps] During scanning don't emit warnings-as-errors that are ignored with diagnostic pragmas.

Before the fix the scanning would fail with
`-Werror,-Wnon-modular-include-in-module` despite the warning being
suppressed in the source code.

Existing approach with `-Wno-error` is not sufficient because it negates
only general `-Werror` but not specific `-Werror=...` and some warnings
can still emitted as errors. Make the approach stricter by using `-w`
flag and ignore all warnings, including those upgraded to errors. This
approach is still valid as it doesn't affect the dependencies.

rdar://101588531

Differential Revision: https://reviews.llvm.org/D138252