Introduce the LogicValue protocol, which is used to establish when a
particular type can be used as a logical value in a condition. Make
Bool conform to this protocol.
Rename the Builtin.i1-returning Bool.getLogicValue() to
_getBuiltinLogicValue(), because this is meant to be a builtin hook
for the compiler. We can add more underscores here if needed to hide
this further from users.
Move the implementation of _getBool() into C code. It was performing
an 'if' on a Builtin.i1, which does not (and should not!) conform to
the LogicValue protocol. Once more oneof IR generation comes online,
we'll be able to write both this and _getBuiltinLogicValue in Swift
directly.
Swift SVN r5140
