Revert "[NFC] Make FPClassTest a bitmask enumeration"

This reverts commit 2e416cdd.

Breaks the GCC build:

In file included from /home/npopov/repos/llvm-project/llvm/include/llvm/ADT/FloatingPointMode.h:18,
                 from /home/npopov/repos/llvm-project/llvm/include/llvm/ADT/APFloat.h:20,
                 from /home/npopov/repos/llvm-project/llvm/lib/Support/APFloat.cpp:14:
/home/npopov/repos/llvm-project/llvm/include/llvm/ADT/BitmaskEnum.h:66:22: error: extra qualification not allowed [-fpermissive]
   66 |   template <> struct llvm::is_bitmask_enum<Enum> : std::true_type {};          \
      |                      ^~~~
/home/npopov/repos/llvm-project/llvm/include/llvm/ADT/FloatingPointMode.h:223:1: note: in expansion of macro ‘LLVM_DECLARE_ENUM_AS_BITMASK’
  223 | LLVM_DECLARE_ENUM_AS_BITMASK(FPClassTest, /* LargestValue */ fcPosInf);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/npopov/repos/llvm-project/llvm/include/llvm/ADT/BitmaskEnum.h:67:22: error: extra qualification not allowed [-fpermissive]
   67 |   template <> struct llvm::largest_bitmask_enum_bit<Enum> {                    \
      |                      ^~~~
/home/npopov/repos/llvm-project/llvm/include/llvm/ADT/FloatingPointMode.h:223:1: note: in expansion of macro ‘LLVM_DECLARE_ENUM_AS_BITMASK’
  223 | LLVM_DECLARE_ENUM_AS_BITMASK(FPClassTest, /* LargestValue */ fcPosInf);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
[43/4396] Building CXX object lib/Supp...iles/LLVMSupport.dir/CommandLine.cpp.o