This initial implementation just delegates from SILGenFunctionBuilder to
SILFunctionBuilder. I was going to start transforming verbose uses of
SILFunctionBuilder into higher level APIs on SILGenFunctionBuilder, but I have
run out of time.
This is a good incremental step forward that will let me hide the constructor of
SILFunctionBuilder after I update the optimizer and thus ensure that
SILFunctionBuilder is only used through appropriate composition APIs.
rdar://42301529
