[NFC] Make FPClassTest a bitmask enumeration

This is recommit of 2e416cdd, reverted in 8555ab2f, because GCC
complains on extra qualification. The macro LLVM_DECLARE_ENUM_AS_BITMASK
does not specify llvm:: anymore, so the macro must occur in the namespace
llvm. Documentation updated accordingly. The original commit message is below.

With this change bitwise operations are allowed for FPClassTest
enumeration, it must simplify using this type. Also some functions
changed to get argument of type FPClassTest instead of unsigned.

Differential Revision: https://reviews.llvm.org/D144241