[IncludeCleaner] Support mapping files similar to IWYU

[cmfcmf]

It would be great if IncludeCleaner could support mapping files similar to IWYU: https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/IWYUMappings.md. For example, during Chromium development, clangd will often suggest the wrong include paths for header, like gtest/gtest.h instead of the Chromium-specific testing/gtest/include/gtest/gtest.h or absl/types/optional.h instead of third_party/abseil-cpp/absl/types/optional.h. The conclusion in this chromium.org thread was that we would need to find a way to configure clangd's IncludeCleaner to achieve better DX here.

[johngladp]

I'd also request this feature, including the "ref" entries.

For example: including vulkan.h doesn't satisfy IncludeCleaner currently and we get asked to include vulkan_core.h which is a file included within vulkan.h and an invalid suggestion (you're supposed to include vulkan.h)

Similarly with gsl.h, we're asked to include gsl_pointer.h directly which isn't a suitable requirement as you're supposed to include gsl.h.

In both of the above examples, both are external projects so we can't add // IWYU pragma: private ... statements to those header files so a mapping file is required ideally.