[ADT] Alternative way to declare enum type as bitmask

If an enumeration represents a set of bit flags, using the macro
LLVM_MARK_AS_BITMASK_ENUM can make operations with such enumeration more
convenient. It however brings problems if the enumeration is non-scoped.
As the macro adds an item LLVM_BITMASK_LARGEST_ENUMERATOR to the
enumeration type, only one such type may be declared as bitmask. This
problem could be solved by convertion of the enumeration to scoped, but
it requires static_casts in new places and the convenience can be
eliminated.

This change introduces a new macro LLVM_DECLARE_ENUM_AS_BITMASK, which
allows non-invasive convertion of an enumeration into bitmask. It
provides specialization to trait classes, which previously were built
based on presence of LLVM_BITMASK_LARGEST_ENUMERATOR in the enumeration.
The macro must be specified in global or llvm namespace because the
trait classes are declared in llvm namespace.

Differential Revision: https://reviews.llvm.org/D144202