Use Xcode's clang instead of Chromium's

This matches what client apps will be using, so will hopefully reduce
errors due to version mismatches. (E.g., clang 5.0 mangles Objective-C++
function names containing Objective-C protocols differently from earlier
versions, which can lead to linker errors.)
Remove an unsupported warning flag
Don't try to use Chrome's clang plugins with Xcode's clang