Core/Spells: Implement SPELL_ATTR4_FIXED_DAMAGE

Spells with this attribute will ignore damage taken auras except for mechanic damage taken (trauma, etc) and will ignore resilience because the damage caused by those spells is based off another spell' damage
Merge pull request #12201 from Unholychick/patch-2

Core/Spells: Implement SPELL_ATTR4_FIXED_DAMAGE
Merge branch 'master' of https://github.com/TrinityCore/TrinityCore into ghbb

Conflicts:
	src/server/game/Entities/Unit/Unit.cpp