Adding IRBuilder::getHalfTy() to IRBuilder.h

[Puyan Lotfi]

I noticed that we don’t have a getHalfTy() method on IRBuilder, but we have one for float and double.
So I figured it should just be there for completeness (since all the other ones do is trivially call Type::get.*Type). 

Any suggestions or objections?

-Puyan

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