[Format/ObjC] Support NS_ASSUME_NONNULL_BEGIN and FOUNDATION_EXPORT in ObjC language guesser

This adds to the ObjC language guesser a few more common macros used
in ObjC headers. These can help distinguish ObjC headers which
otherwise lack ObjC types from C++ headers.

Contributed by danblakemore.

Tested: New tests included. Ran unit tests with:
  ```
  % cmake -S llvm -B build -G Ninja && \
    ninja -C build FormatTests && \
    ./build/tools/clang/unittests/Format/FormatTests --gtest_filter="*FormatTestObjC*"

  (snip)
  [----------] 24 tests from FormatTestObjC (265 ms total)

  [----------] Global test environment tear-down
  [==========] 26 tests from 2 test suites ran. (270 ms total)
  [  PASSED  ] 26 tests.
  ```

Reviewed By: MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D151578