Symbol visibility warnings in core expressions code

[friedbunny]

iOS builds on master (such as this one, as of ca9d9bb) are currently showing a bushel of similar warnings in newly added expressions code about how, “direct access in [function] to global weak symbol [symbol] means the weak symbol cannot be overridden at runtime”:

⚠️ ld: direct access in function mbgl::style::expression::ArrayAssertion::operator==(mbgl::style::expression::Expression const&) const from file '/Users/distiller/project/build/ios/Debug-iphonesimulator/libmbgl-core.a(array_assertion.o)' to global weak symbol typeinfo for mbgl::style::expression::Expression from file '/Users/distiller/project/build/ios/ios.build/Debug-iphonesimulator/test.build/Objects-normal/x86_64/MGLFillStyleLayerTests.o' means the weak symbol cannot be overridden at runtime. This was likely caused by different translation units being compiled with different visibility settings.

/cc @anandthakker @1ec5

[1ec5]

a bushel of similar warnings