[Expr] Fix `TestExprOptions` after r348240 on MacOS X

Summary:
r348240 assumes that an expression contains the Objective C option if
Objective C Runtime is found. But on MacOS X it seems that the test application
process always contains Objective C Runtime, so the test fails when it assumes
that the language is C++ only. Skip this part on Darwin.

llvm-svn: 348250