LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE() pulls the operator overloads used by LLVM_MARK_AS_BITMASK_ENUM into the current namespace.
Declared in <llvm/ADT/BitmaskEnum.h>
#define LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE()
Suppose you have an enum foo::bar::MyEnum. Before using LLVM_MARK_AS_BITMASK_ENUM on MyEnum, you must put LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE() somewhere inside namespace foo or namespace foo::bar. This allows the relevant operator overloads to be found by ADL.
You don't need to use this macro in namespace llvm; it's done at the bottom of this file.