clang-format: Fix many Objective-C formatting regressions from r289428

r289428 added a separate language kind for Objective-C, but kept many
"Language == LK_Cpp" checks untouched.  This introduced a "IsCpp()"
method that returns true for both C++ and Objective-C++, and replaces
all comparisons of Language with LK_Cpp with calls to this new method.

Also add a lot more test coverge for formatting things in LK_ObjC mode,
by having FormatTest's verifyFormat() test for LK_ObjC everything that's
being tested for LK_Cpp at the moment.

Fixes PR32060 and many other things.

llvm-svn: 296160