Revert "[NFC] Make FPClassTest a bitmask enumeration"

This reverts commit e7613c1d.

GCC issues an error:

In file included from /home/buildbot/as-builder-4/lld-x86_64-ubuntu-fast/llvm-project/llvm/unittests/ADT/BitmaskEnumTest.cpp:9:
/home/buildbot/as-builder-4/lld-x86_64-ubuntu-fast/llvm-project/llvm/include/llvm/ADT/BitmaskEnum.h:66:22: error: explicit specialization of template<class E, class Enable> struct llvm::is_bitmask_enum outside its namespace must use a nested-name-specifier [-fpermissive]
   66 |   template <> struct is_bitmask_enum<Enum> : std::true_type {};                \
      |                      ^~~~~~~~~~~~~~~~~~~~~
/home/buildbot/as-builder-4/lld-x86_64-ubuntu-fast/llvm-project/llvm/unittests/ADT/BitmaskEnumTest.cpp:30:1: note: in expansion of macro LLVM_DECLARE_ENUM_AS_BITMASK
   30 | LLVM_DECLARE_ENUM_AS_BITMASK(Flags2, V4);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~