Previously, the Objective-C heuristic failed to detect
headers which #imported Objective-C system framework(s) and declared C
functions on top them.
This extends the heuristic to look for statements of the form:
and check if "Foo" is one of the known Objective-C system frameworks.
Test Plan: New tests added. Ran tests with:
% make -j12 FormatTests && ./tools/clang/unittests/Format/FormatTests