[Dependency Scanning] ClangImporter: Adjust to "[clang][modules] Serialize CodeGenOptions (#146422)"
`ClangImporter::create` overwrites the
`clang::CodeGenOptions::OptimizationLevel` setting based on
`swift::IRGenOptions::OptMode`. Respect the swift optimisation mode in
`ClangImporter::getBridgingHeaderOptions` so that the bridging header
PCH, which is emitted using these options, is emitted and later consumed
using the same clang optimisation level. Otherwise, the mismatch will be
caught and reported as an error when reading the PCH.
See https://github.com/llvm/llvm-project/pull/146422.