In clang 9, the signature of `registerPPCallbacks` changed and it no longer provides us a `CompilerInstance` [0]. I couldn't find a way to reach the `CompilerInstance` from the new structures, so I've inserted a level into the class hierarchy that mimics the old functionality.
With this patch, clang-tidy compiles with either clang-8 or clang-9, so we can land this independently in preparation for bug 1573211.
[0] For reference: the new signature was introduced in bbc89dcb29, the old overload was marked deprecated in e9087fe75c and removed in ee737a84d7.
Differential Revision: https://phabricator.services.mozilla.com/D46702
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extra : moz-landing-system : lando
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