Improve ThrowHelper

[benaadams]

• ThrowArgumentOutOfRangeException inlined too much so changed to throw directly which stops it being inlined (and its inner Enums)
• Renamed to ThrowArgumentOutOfRange_IndexException so its clear what is out of range when called.
• Aggressively inline IfNullAndNullsAreIllegalThenThrow as is above inline size; so its either entirely dropped for reference types; or reduced to the single non-inlined call for boxed value types.
• Use Enum.ToString for look up as it is fast rather than the manual switch statement translation of the enum to a manual string of the same which is more error prone.

[jkotas]

LGTM. Thanks!