[clang-format][NFC] Change EXPECT_EQ to verifyFormat or verifyNoChange

Replaces some 600 EXPECT_EQ() to verifyFormat() or verifyNoChange() in
FormatTest.cpp because the former neither checks stability of formatting
nor tests formatting C++ as Objective C.

Also marks dozens of unstable test cases with FIXME comments.

Differential Revision: https://reviews.llvm.org/D159051